I think it's fine to have an opinion on purchases made like that. Sometimes I wish my mom had been a little more in my face about things like that (wouldn't have felt that way at the time, of course).
Maybe you've now realized your house rules need a little tweaking. Since you are worried about him, and since it seems he responds well to house rules, why not help him more? Savings (might want to label what that savings is for, so he knows that it can be spent in certain emergency circumstances), but also "put this aside for future rent, put this aside for food purchases, put this aside for future insurance", etc etc? That way he has REAL knowledge of what is left for "fun" spending? Assuming you're not doing that, of course.
And I have to mention...
He brought a nook book reader. problem is he is not a big reader in fact because of his syndrome he really doesn't like reading except for a few books.
DH works for the Kindle team, and they are finding that there is something about the e-book readers that REALLY works for people who have a hard time reading. DH is actually an excellent example of it; he graduated from high school with nearly a 4.0, but had basically only ever read one book, and used and re-used that book for English every year, for History, etc. Reading is very very slow for him, and it's hard for him to get into a book enough that he WANTS to keep going.
But ever since he got a Kindle, he has slowly started reading faster. Not sure why, but he's become a "reader" at 40, when he never was one before. So it's entirely possible your son will be one of the new, good "statistics" with the e-book readers.
Perhaps if the device, which i'm assuming he purchased, is truly only used for a few days it could be either returned for credit or sold to recoup some value, as opposed to being placed in a junk drawer.
Good point.
This is your opportunity to get him into reading. You just need to find that "gateway" book. I would suggest a graphic novel. Some of them are extremely good.
Good gateway! And in case the OP doesn't know, "graphic" doesn't mean naughty, LOL. It's somewhere between a comic book and a book-book.