Shari, with all that's been happening at school especially with the way they mishandled him, it's not at all surprising that he is reacting this way. From my experience, it would explain the lot, frankly.
Your husband swatting his rear - that shouldn't be happening. I'm not saying this from an "anti-spanking" moral stance, but from experience of what works/doesn't work with the range of problems you describe.
And you're thinking - "so you take away spanking, and what do we have left?"
I know, in some ways it makes it trickier, but it does work better in the long ru. As they get older you have to wean off spanking anyway because one day they will be bigger than you/stronger than you and it won't be pretty. Better to train them while they're smaller, that hitting is not a good thing.
And that's where the reason lies - if FOR ANY REASON your child is socially inept, then they cannot learn how to behave just by being around other people; it's just too complicated. They need good role models to model for them, the correct way to behave. You need to SHOw him by how you behave to him, the way for him to behave in return. It's in how you speak to him, how you behave to him, how you interact with others also, but how you are to him is how he will be to you. Eventually. if he gets confused by mixed messages, then you get the hissing spitfire. If you continue to stay calm and quiet, he will learn that the hissing spitifre is not the way to go, because nobody else does it or even comes close.
That's why hitting has to be out - if you're trying to stop him hitting, then you have to stop anyone hitting him. Otherwise, he gets a mixed message.
That problem teacher filled him with mixed messages. She has caused you a great deal of the problems, and by mishandlnig him the way she did, she triggered and then escalated the problem behaviours that finally gave her the leverage to get him kicked out. And now you're dealing with the escalated behaviour at home.
Think about how he is feeling - he has been kicked out, under circumstances that he feels are unfair. Somewhere deep down he knows he was bad; somewhere deep down he knows tis teacher was bad also. But he has to find an explanation - and the only explanation he can find, is that it's OK for the teacher to do it, but not OK for him, therefore it's him, HE is bad intrinsically, so he may as well just let it all hang out.
His self-esteem right now is probably rock-bottom. He is angry, he is frustrated, he is afraid, he is very hard on himself but also angry very easily. He probably feels set up for failure, that he just can't get anything right so why bother?
I'm not saying that therefore you should give in to him all the time and let him walk all over you. Not at all. But I think you need to go back to basics, as if he were a kid who forgot all his table manners, forgot all his toilet training, and you had to start form scratch.
Get in touch with him inside. Always try to keen in mind, why is he doing this? What is driving him? Then target the driving force, rather than the current behaviour.
For example, difficult child 3 will sometimes get frantic at me. "Come on, Mum, hurry up! You're so slow! We'll be late! We're lost, we'll never get there on time! Why aren't you using a map? Where are we going? I don't want to go any further, I think we should turn round now ands go home. Why are you ignoring me, Mum? You never listen!"
None of that is true rudeness. ALL of it is anxiety-driven. Most parents would scold tat sort of talk, would really get annoyed. But if you do, it makes it worse because it drives the underlying anxiety to an even higher level.
What works - reassurance and persistence. Sometimes if the anxiety is too high, we stop and pause. We talk, I explain WHY it's not so bad after all. But sometimes we just need to finally get to where we're going, on time and safely, for him to feel safe again.
The example above was difficult child 3 on the way with us to his end of school year presentation day, where he was due to receive an academic award. After the awards ceremony was over and everybody was leaving, difficult child 3 started up again, this time hassling over the number of people in the elevator. He had to control who went in, when, for how many floors and so on. He counted heads to make sure we weren't over the limit. He fussed at every odd sound in the drive mechanism, he was loudly anxious. Thankfully the school staff are used to him now, even though a lot of them haven't met him often. They accepted what he was doing and just carried on. Nobody reacted to the apparent rudeness.
With wee difficult child, stop thinking of him according to his age. In some ways you need to think of him as a baby, in other ways like an adult.
Treat him with the same respect you would show to another adult. If a flatmate had grabbed the remote to switch off the TV, how would husband have reacted? The logical way to react with another adult, is to either go get the remote, turn the movie back on but keep hold of the remote, or go find another TV to watch.
But let's analyse difficult child's behaviour. He's been hitting husband. Hitting others. Tormenting the cat. Sounds to me like a combination of anger, frustration and not knowing what to do with himself.
So, some suggestions:
1) Don't hit. He's using hitting as a way to react, so all that you hitting him is doing, is reinforcing this. In other words, hitting is not only not helping, it's now part of the problem.
2) Try to anticipate the need. When you notice his behaviour starting to escalate, deflect him. Removing the cat was good (for the cat, especially) but you needed to replace it with something physically active. How is the weather outside? A suggestion is to put a tennis ball inside a stocking (or similar) then show difficult child how to stand with his back against a brick wall and swing the tennis ball in the stocking so it bounces off the wall beside his head, arms, legs, whatever. It sounds loud (especially with your head against the wall) so it feels a little like you're hurting yourself, but you're not; there is an adrenalin rush/release from doing this, and it is also good exercise. Put this into place ahead of time. Explain to difficult child that this isn't a punishment, it's an alternative activity. He can choose to do this himself at any time, or you will ask him to go do it at any time if you see the chance he may benefit from it.
Warning: Do not do this on a gyprock/plaster wall. They break.
Another alternative is the jogging trampoline - they can be used indoors in bad weather. We had one, we'd send difficult child 1 to do 20 jumps on the jogging trampoline, to burn off excess energy. When not in use the legs unscrew and the tramp slides behind a cupboard for storage.
Another option if it's hands you see needing to do something - stress balls. Or similar squishy toys. difficult child 3's school sends him these occasionally, with the school logo on them. I wonder if they're trying to tell us something?
But it comes down to - anticipate.
3) Deal with the source of the problem, as best you can. For example, right now difficult child is feeling pretty awful, a failure and unloved. He needs some successes, but instead all that he seems to be getting right now is discipline and trouble. He won't be fully understanding why the discipline, because once a problem escalates to the level you describe, he can't mentally go back to identify why he is in trouble, all he knows is that everybody is being mean to him. He feels justified. That is not good because we all know, he is NOT justified to behave the way he has.
You need instead to de-escalate here possible, so he has a chance to remember the incident in perspective and then learn from it. He actually has MORE chance to learn from it, with LESS punishment. That's because the more punishment he gets when angry, the more he sees that punishment as unfair, therefore the lesson is lost. So don't punish him when he is angry. Instead, just keep him safe and yourselves safe. This means time-out can change from punishment, to a safety zone. It's not Supernanny stuff at all. I don't have a time limit for having a kid in his room; I let the kid out as soon as he is calm, especially if he apologises. But I found with difficult child 3, I had to not insist on the apology if he hadn't understood that he had done the wrong thing. Any apology insisted on, would have been meaningless.
difficult child 3 actually learned to apologise, by hearing me apologise to him (where it was appropriate).
This is where it can be tricky - when you have to keep at the same time, the "my child is an infant" and "my child must be treated with adult respect" in your head at the same time.
You've all had a really lousy time lately. It's no wonder you're all fractious, fed up, not too patient with one another. There must be some resentment of difficult child in you, for causing this much strife. And he probably resents you, too, for not making it all better. It's not going to be easy. But if you have an idea of what could be behind it, it might be easier to deal with.
Something to consider in this - I found, when difficult child 3 was needing my attention (any of the kids, really, difficult child 3 more than most) I had to stop my own entertainment and put it aside to deal with the kids. Within reason, but the kids least capable of being reasoned with are the ones least capable of waiting. And you can't teach such kids patience by making them wait. (might work for some, but not our Pervasive Developmental Disorder (PDD)-ish difficult children!).
So you need to learn and put into practice - you will enjoy the movie ore, if you've got difficult child's needs met first. It's much harder to enjoy the movie if there's a kid in your face saying, "I want a drink of water NOW!" (or similar). As time moves on, you can begin to teach them to wait a few seconds, then a few seconds more.
Catch him out being good and praise him. If he walks past the cat without hurting it, praise him for being good. If he walks past the cat and strokes it gently, praise him for being loving and gentle. If he hurts the cat, remove the cat but don't say anything.
You're currently in desperation mode, thanks to circumstances beyond your control (and well beyond difficult child's control). Desperate situations call for short-term desperate measures.
If you can, get husband to lurk here/post here too. It really helps my husband ("Marg's Man") to be on the same page.
Marg