He sounds really anxious. And when he relaxes he will be able to let you go more, I suspect.
You've got two things in your OP which would make anyone anxious, and so presumably, on at least one front (the divorce), things will eventually settle, and should get better, I would hope.
And you say you're fighting for him at school, which means that although he's doing a sterling job, and going in every day, which must be really hard, he isn't having a great time there. Do you think there's any hope of this getting better? If it's tough for him, does he have an EHCP?
I have a dd who cannot manage year 2. And I don't think it's unusual for our kids to fall apart as the demands of school ramp up. He's really doing well to stick it if he's really anxious at the minute.
I would see what else could be done on the school front to make his life easier, and go from there.
You have my sympathies. It's very hard to not get a break at all.