Sounds like you're on the right path so far.
On the subject of "You're too easy on him," what he needs is consistency, as well as what other people might see as a light touch.
A quick summary of how the Explosive Child book works - it puts a lot of control back onto the child, in terms of them needing to learn self-control. But you are the guide, the facilitator, the person the child learns to come to for support and assistance. You stop being the authority figure and lose "because I said so," and it becomes, "We need to think about this. If you do X, then Y will happen and that may not be good. Let's see if we can find a better way for you to get what you want, and still do it safely and the right way."
You need to make a list of what you want to change about his behaviour. Prioritise the list, put at the top the dealbreakers.
Then look at the list again - you've probably got about a dozen things in the dealbreaker column. And they have to be moved to a "we'll work on the top three" column.
The dealbreaker column - the only things that you can have there, are immediate safety and school attendance. Ini our family, even school attendance is not on the list because we use a form of home schooling.
For example - if you know your child is likely to have a meltdown if you lay hands on him, but you don't want to see him flattened by a truck, you will grab him if he's about to dash onto the road in front of a truck after a ball. Grab first, explain later. But if you're in a paddock somewhere and he runs off after a ball that is rolling in the grass, let him go. Only go after him to bring him back, if he gets in a situation of immediate danger.
Similarly - we often find ourselves standing blocking the doorway saying, "You're not going outside without your coat." And this will provoke a meltdown and oppositional response in a kid whose main aim is to get outside to play. Once it gets to this stage, the child is LESS likely to come back later and admit to feeling cold. But if you either let the child go, then have the coat at the ready when he comes back (he has no reason not to - he hasn't lost face) then YOU have won - he is wearing his coat, plus he learned the consequences of going out in the cold without his coat (you get cold).
Or you can say to him, "Which coat do you want to wear when you go outside? Do you want your red one or your blue one?"
This gives him choice and a feeling of control, but it also gets what YOU want, which is him wearing a coat, any coat. And you have become the facilitator for him, not the obstacle to his play.
With you and your husband separated, this complicates the picture. But what your husband does, really shouldn't matter here. You do the right thing and your relationship with your son should improve. Of course other people not on the same page will lead to the oppositional behaviour continuing, especially with those other people. But if you make allowances for him being grouchy soon after you get him back, and give him the space to re-acclimatise to you, things should begin to turn around. For you. And when your husband complains that things are getting worse for him, lend him the book.
it's also possible that his dad lets him do whatever he wants, which would be why they get on with no problems. This would cast you as the ogre and disciplinarian, which aggravates the ODD stuff for you. So change your methods and mind-set following the book and this should begin to turn around for you.
One thing to bear in mind - your son is not trying to be a pain. He undoubtedly wants to be seen as good, as helpful, as someone you enjoy being with. And when it clearly isn't working out, it upsets him too and he gets grouchy feeling, "Why bother?" That is when you really see the Mr Hyde emerging.
These kids do try to hold it together at school or with people they feel less secure with. It's the ones who they feel safest with, who see the worst behaviour, as a rule. And it's not always directed at you, nor is it usually meant as "I hate you," Even if that's what the kid says. These kids get very frustrated, very angry, very impulsive. The world is a confusing, contradictory minefield for them and they need a guide to help them along the path. You are that guide.
So you don't have to be the ogre. Not at all. There are other ways to still get what you want. But you may have to modify what you want, to expectations more achievable for him, at least for now. baby steps.
It really does work, and it can work fast. But it's not a cure. He needs to change himself long-term, and this will need expert help. He will be trying to get it right, but it comes much more difficult for these kids.
Stick around, let us know how you're going.
Marg