I think this is natural, and I’m a bit like this also sometimes. Life with young kids and jobs feels so chaotic, sometimes all I want to do in the evening is sit and watch tv with my dinner on my lap 😩 But you do have to make effort or your relationship will fail.
OP I agree, you’re not a moron, he isn’t a monster. He dropped the ball and stopped making any effort, he took you for granted, he probably did resent you and he wasn’t a good partner. Also just not a good fit for you and your son. How he is behaving now is awful though.
I tried to put myself in his shoes - if I was thinking about having to go back to a 1.5 bed flat with three neurodiverse children on my own, work a full time job, and do EVERYTHING on my own - the bills, cooking, cleaning, all pick ups and drop offs etc I’d probably be majorly panicking and wanting to get myself back in a ‘family’ set up in a house. With another adult around and to help with the bills etc. Because that does sound really overwhelming!
But it’s not the OPs problem, and he is casting around for anything that might change her mind. But I don’t think the ‘let’s have casual sex as I’m going to be with other people if you make me move out’ messages are at all forgivable. It’s ALL about him.
Sorry OP. I think you’ve been amazing also to be so decisive and just get it done. I certainly have always found it very hard to end long term relationships