I think DH is probs putting his head in the sand as he's a bit sensitive about the similarities.
I've been wondering since your first post of I should say this, but my instinct (when you described your DH's insistence on responding with traditional behavioural approaches and him seeming overwhelmed from your description) was that your DH may also be undiagnosed autistic, or have autistic traits.
I say this from my own journey of having realised with a horrible shock after my two were both dx that I am likely autistic too, and that's finally the answer to what's been going on for me the past 40 years. It's both an absolute relief, lightbulb moment, and the most painful upsetting thing ever. (I'm talking about my own, personal experience, other autistics will have a wide variety of experiences.) I'm now a year and a half into a three year wait on the NHS adult autism pathway.
In the past few years autism has become a special interest of mind, first through reading up for my kids, and then for myself. I believe it to be largely genetic, with some environmental influence (before birth). Nowadays when I meet a new autistic child I find myself scanning both parents for traits, curious as to which parent may be autie too! (Not all are, but usually there's an uncle/aunt/grandparent if not a parent.)
It's not easy, at all, for your DH and MIL to have questions, echoes, things rising up in their minds due to seeing how your DS is developing. My DM mentally fought hard against it when my DS was dx. But later she told me she had always worried about me as a child and hadn't known how to help me. She said if they'd known about autism she thinks I'd have been dx too. There was a lot of guilt and processing for her (on her part, not a projection from me) .... the truth is, I've always always know that everyone found me "difficult ", from a young age, and that whatever I tried I couldn't fix it. If you can imagine coping with the level of difficulty your DS has now, yet completely without any concept of neurodivergence and different parenting possibilities other than traditional behavioural approaches.... well, you can imagine how your MIL, for example, might have felt towards your DH. And how your DH may have absorbed it all too. That is not easy stuff for either of them to acknowledge or come to terms with quickly, so perhaps the old default position of "just try harder/be firmer" is still at play. It's also possibly a psychological defence against the pain of accepting DS's differences. It's just as you say, we long for our kids not to have hard lives.
This has turned into an essay! With no practical suggestions for how to help your DH move on from his current approach to your DS. But trying to share what might lie behind his current approach.