Most of my friends from my HS and myself had out college’s paid for our parents. I do not know a single one that had to pay them back for this, so your generalization about it seems odd. Ever hear the phrase “Dont lend out more than you can afford to lose?” Well this is why that phrase exists. It’s unlikely you have leverage to force anyone to pay you back, from what you have written it sounds like your son is independent so it’s not like you can kick him out of your place or anything. If you want to like sue your son, go for it. If not, then you need to just talk to him and see whats up, and form a plan.