Don't spank him! Whatever you do, spanking for toiletting issues is the wrong thing to do.
I suspect he overheard enough of your argument to want to please you. Sounds like he got the message.
We went through this with difficult child 1, only with him, he wouldn't do it in his Pull-ups either. It was a week between poops, he would be frantic. He was too terrified to sit on the toilet, even the potty was purgatory for him and it took our weekly visits to his grandfather who would bully it out of him (no spanking, and with hindsight I don't think it was right).
Things we tried with difficult child 3, to head off this problem - we used bribes. We got those mini-M&M boxes, the ones you get in a multi-pack, and we broke up the task. We'd also tried bribery with difficult child 1, but his problem became so bad, so quickly (what with refusal to do it ANYWHERE, for a week) that we didn't have time for bribery to work.
So first, we offered a bribe for sitting on the potty. Just sitting. difficult child 1 couldn't do it bare-tailed, so we gave him a bribe just for sitting on the potty, clothed. His rear end fleetingly touched it and then he said he had earned his reward. Strictly speaking, he had. So next time, he had to sit for a whole minute, fully clothed. Then he had to sit with bare behind. And so on.
difficult child 3 was not a problem with sitting on it, he simply didn't get the connection about what he had to do. He also didn't have the language skills to tell us, or to understand very easily when we talked to him. difficult child 3 went straight to the toilet, not the potty. We blu-tacked a box of mini M&Ms to the wall above the toilet, they were his for actually producing something in the toilet. At first it was small pellets - "rabbit poo", we called it.
Then he got a box for using the toilet AND having clean Pull-ups. Then it was a box for going a week with clean Pull-Ups and using the toilet.
Bladder training - that was a whole 'nother kettle of fish.
We still had accidents for quite a while - years - but he had the idea of what to do. Sensory Integration Disorder can occur independently, but is fairly common in autism. So are some of the other things you describe about him. In Australia we don't get those labels as a separate diagnosis, we get them as part of the autism diagnosis. So I'm not really used to them as often discussed here. For example, we know all three of my younger kids have sensory integration issues, face recognition problems and obsessive/compulsive issues, but these (for us) are all part of the whole constellation of things we have to deal with, part of the autism spectrum component each child is dealing with.
There is a saying, "You catch more flies with honey than with vinegar," and this is especially true in autism. Rewards and incentives will work a lot better than punishment - for these kids, the world is already punishing them. But they do want to please you, and they do try. Often what you're asking of them is still beyond their ability to give you, and here even rewards will not work, you just have to be supportive and wait until they are ready. To punish for an inability to comply, is like punishing a newborn baby for failing to clearly enunciate requests; or like punishing a blind child for not copying accurately from the blackboard.
Read the book recommended. Try and get husband to read the book. if he can't/won't, don't hassle, just explain it to him. This will help you get a better grasp of it yourself. The techniques described do sound like you are spoiling the child and the usual discipline techniques which have worked for you for years, will be what your husband wants to use. But these techniques, while considered effective for most kids, not only don't work in some cases but they can make a problem worse.
Switching methods won't spoil a child. You can only spoil a child be being inconsistent. The new method seems to be handing all control to the child but it actually is not - it is giving the child SOME control but in the process it is teaching self-control. It is also teaching that you are not the obstacle or barrier to what he wants, you are the facilitator. You don't work on ALL discipline issues, you choose a few and ignore the rest. For now. The only exceptions are immediate and urgent intervention needed for safety. Otherwise, you try to negotiate with the child but back away before the child goes into meltdown. Soon the child realises you are trying to help and also trying to help prevent meltdowns.
If you seem to be making absolutely no progress on something, that's when you need to consider - am I trying to work on something the child cannot yet control? If so, change priorities.
The Ross Green method will also work on other children. It builds independence and capability, as well as a sense of personal responsibility.
There are a number of apparent behavioural problems in autism, but the most important one for you and husband to realise, is that he will model the behaviour you show to him. Unlike 'normal' kids, an autistic person will not pick up appropriate social behaviour purely by osmosis. They need to be taught, as if you are teaching them their times tables, or the alphabet. They also need to learn by example. They will not distinguish between individuals - they are as purely egalitarian as any individuals in the world -everyone is equal. There is no status - child, adults, teacher, pupil - all are equal. Show him respect and he will show others respect. Use sarcasm, punishment, a harsh tone and he will use the same in his responses, because you are modelling these for him.
How does difficult child interact with a baby? I'm betting he tries to treat the baby as someone with his own level of understanding of the world. My difficult child 3 was an early reader (without the comprehension to go with it). When he was 11, he was in a hospital waiting room looking for a book to read. Knowing he HAS to read aloud, I suggested he read to a baby (about 6 m o) also present. The baby was getting a bit unsettled. difficult child 3 sat down beside the baby's bassinette and held up three Spot books. "Which one do you want me to read?"
The baby's hand waved aimlessly, which difficult child 3 took to be a preference selection. he said, "Alright, this one then." and he read the book, making a point of showing the pictures to the baby and asking the baby questions about what he was reading (which we always did, to expand his own understanding of the text). Of course, the baby didn't have a clue, but still settled down because someone was paying him attention and he was happy to look at difficult child 3 talking to him. It was really sweet, but demonstrated how difficult child 3 was still lacking theory of mind.
difficult child 3 now can show some theory of mind if he thinks hard about the problem, but it has been laboriously taught, it's not automatic for him.
Also, keep an open mind on the diagnosis - give him credit, do not treat him as if he is delayed; neither should you push him too hard. difficult child 3 'failed' his first IQ test at age 4 but we have since been told his IQ tests above 140. Similarly, difficult child 1 failed his first test. We were told, with both boys, that they were retarded. No way! It's just very hard for an autistic kid to be tested properly, especially when younger. Their language delay issues get in the way, big time. But language often improves, which totally changes how they respond to an assessment.
If he shows interests or gifts anywhere, encourage him. Support him, praise him. Love him and tell him you love him. It's the way they learn to express love, by experiencing it for themselves.
Marg