Even assuming that we could all somehow arrive, through the magic of the internet, at a diagnosis or explanation, what difference would it make?
He's a young adult now, and the OP has no more influence over diagnosis or treatment for his 'condition' (whether that be a physical, emotional, or psychological one) than I do for my sister-in-law or my third cousin. Possibly she has less, given that the young man's mother is not inclined to accept advice or intervention from the OP.
It is not the responsibility of the OP to deal with the DSS's issues. This actually is about the Santa train, because that is the thing which OP can reasonably expect to influence. The reasons behind it are not for her to address; she can't, and she doesn't have any legal or moral or emotional obligation to do so.
OP, you are quite right that it is about looking for ways to deal with your DH, and getting to the point where you are saying "I'm taking the little ones to see Santa", and be confident of being supported in that by your DH. Myself, I'd be inclined towards recommending rows (with DH) and the putting down of feet, if's what it takes...but I also know that it isn't always that simple.
What if next time (I mean next comparable event, obviously) you don't discuss in advance? You get the tickets, and present it as done. Dou don't ask for permission, you don't have to.