The closest I come is A 159 with matching shape and colors. It's phentermine, a type of speed used for weight loss (not effectively for long-term results). This was part of the controversial phen fen a few years ago. If you take enough of it, you could get a buzz off of it if that's what it is. R 59, on the other hand, is lorazepam, a benzo. You could get a buzz off that, too, very sedating relaxant.
Know any pharmacists? You could take it in and ask. You don't have to say where it came from. But I think I'd be concerned about finding pills in my son's backpack. :-((