Massage therapy school is a trade school, so he'll need a high school diploma or GED to enroll. Also, most states will not license anyone under 18 for legal reasons.
That means he can't go to massage school yet and he can't do massage for money, but it doesn't mean he has to forget about massage altogether! A lot of massage schools offer classes for the general public that are not geared toward licensure, but would give him some great tools to build on with he massages his friends and family. He can try to get a job in a massage environment, like a spa or a corporate massage chain like Massage Envy (they're evil and horrible, in my opinion, but they do exist). That would let him work in a massage environment and pick up a lot of info from the MT's while he finishes high school.