Im sorry I didnt mean to offend. You mentioned one child had PIP, but both had complex needs. Both my children have PIP and complex needs, and both going to uni (although one having to study near home) so its hard to know exactly, as complex is, well, complex. As you mentioned one possibly living independently and potentially not receiving PIP I made the false assumption that this child might have possibly studied. sorry
I would contact womens aid. Also do either of your kids have a social worker? You mention you get 3 hours respite a week - so do you have anyone to advocate on your bnehalf.
Its clear to see that this man is abusing you and he is not going to engage with any discussions or change. You cannot force him to do so, and you say he even controls what you speak about, so it does not sound a positive place for you to be in even if there was any more cash sloshing around. You are right to be concerned, and I think it would be good to start investigating whether, as a single parent, you could access any more help. Indeed if you had more income to play with, via work /UC and the top ups for disability, you may find that you could afford to pay for some help for your child from his PIP.
Could you apply for PIP/DLA for the younger child?
But Id speak to womens aid as a first point of call, to see if you can be helped to get away, and start a process of disentangling yourself from this awful man