Answer:
It is relatively common for boys to develop gynecomastia (man boobs) while going through puberty. It is a temporary condition that usually doesn't need specific treatment since it will go away on its own.
If you are going to buy a son a bra, I would recommend a sports bra that looks as much like an undershirt that you can find. He doesn't need to accentuate breasts and it will probably make him more self conscious.