Many twelve-year-old boys are still young enough to be interested in something like a nice Lego set (that's designed for older kids, as compared to "regular" Legos. Some of the "old-kids" Lego sets have themes that boys are interested in.
Other building sets aimed at older kids may be something he'd like. Also, there are things like sets that help a child that age start a collection of something that would interest him.
Pogo sticks, unicycles, snow boards, skate boards, sports equipment, scooters, and hockey skates are things boys that age may like.
If he has interest in something like sports, animals, space, weather, drawing, or anything else that he gravitates toward - there are books, DVD's, and other related items he may like. Sports items don't have to include the usual equipment that kids often already have (helmets, balls, bats, etc.). There are sports-related items that allow a kid to practice by himself, and there are books about the sport or famous players.
If he plays a musical instrument he may enjoy books or accessories related to that.
A subscription to a magazine he may like could be good.