Just a thought... If your husband has spent a lot of time building a new life, one that your son was not a part of, his emotional connection has likely weaned just as it did to you. Absence does not always make the heart grow fonder.
Now, pre-empting the likely backlash of women reading this by acknowledging that pissed off women would disagree with the next part I write - to each their own....
Many women, intentionally or not, allow the stereotypes to take over about the role of fathers. Fathers are very important, just as much as mothers are. Your ex might have bought into those stereotypes that likely were prevalent in your divorce as well. If he does not feel that he is important to your son, and that your son is not his anymore, his involvement would be naturally low.
Build his relationship over time. Let him see, not be told, that he is important. Thank him without any tone of "it's about time" or "what kind of dad are you".
While I have my kids 3.5 days a week, I have had to fight for that. Many times in the past and present, I have almost let my frustrations lead to me giving up and walking away. Your husband may have had a much better time with you, but when affairs are present, I wonder how much he sees you as a source of judgment? The other side of getting him involved is making sure the small ways you (rightly) feel anger to his treatment of you and your son compared to anyone not be part of the equation.
In the end, he is who he was in many ways. If he wasn't a "kid" person, he still isn't. Many fathers I know become more dad-like over time. He doesn't see his son long enough to do that without YOUR help & the right friends/support. He may also not have got over the special needs of your son. I've heard it is very trying to all parents.
Instead of him being involved because someone said he needed to "help out more", involve him FOR your son. Your son may not say it, but he likely wants or will want a connection to his dad. Did you know that the majority of prisoners were raised in single mother led homes? Think of his involvement as a way to show your love for your son rather than a duty or way to lower your stress.
I'm sure your life is harder than you've posted, but the uphill battle is ALREADY succeeding. Did you think that he'd even do what he's doing last year?
