No. Sorry, but, the answer is no. You don't "owe" him a thing. Look, I know that today we parents are impressed as much as is possible that WE *owe* our kids a college education. And I think it is smart for kids to strive for a college education, and if that isn't their strong suit, technical school and learning a valuable trade, etc. But that doesn't mean you are on the hook to pay for it all. It sucks that it costs so much, but the cost is just going to continue to rise.
Maybe college isn't even the best choice for him right now. I don't know.
In your son's case, he doesn't get more $ just b/c your older kids had outside money given to them. And the fact is that YOU are on the hook for extensive medical expenses for his siblings as well. You cannot pretend those costs are nonexistent. And you must continue to plan with those costs in mind.
He needs to grow up. If he still can't wrap his head around the fact that your older kids were gifted by their Dad (outside of your control) and he thinks *he deserves* likewise from YOU b/c of it, then he has some problems he needs to address. Like, living in reality. He's 18. He knows better. Or he should.
Sounds like he is trying to guilt and manipulate you and you are falling for it. Probably isn't the first time, if he has always played this card before (from your own words). I'm curious what your response was back in those days when he "demanded" the same from you as what your daughters came home with from their dads. If you cave then, then that is where this comes from and shouldn't be a surprise. If not, then he should have learned this lesson already, and I'm not really sure why you sound so willing to cave this time.
No. Just no. You set aside money, use it how you planned (unless circumstances dictate that you cannot). Your minor children, with medical needs/expenses, ARE your legal responsibility to provide for. Your 18 year old son's college education is not. No matter how guilt ridden he may try to convince you to be. Stick to your plan. It's probably one of the most valuable lessons you could teach him right now: That he isn't "owed" anything by anybody. He's 18. If he wants something, time to go earn it. Everything else is a GIFT.