One school of thought, at least in hunter-land, is no tall boots before they have boobs(*), but not sure how this would correlate to a boy. Maybe see what you can find, as others have suggested? Plus see if he has a strong preference for one or the other. My friend with boys outfitted their torsos in shirts, ties, and jackets from JC Penny.
(*)Unless you have a kid who is on the verge or outgrowing, or really has outgrown, their pony. Then you go to jods and paddocks and jack up the stirrups. Kids will even change b/w showing a pony and a horse.
What BFNE said.
A 12yo boy will not want to wear jods. Breeches and boots -- half chaps being a better bargain for a growing boy and paddock boots safer for him and cooler for riding and being around the barn every day. He can save on the breeches too and ride in jeans with the half chaps every day. He's probably ready for a tall boot, but parents may not want to expense that every 3 months! Touch base with local pony club -- they may have a used shop --