Gamertag reported

If you haven't do so already, you would need to file a complaint on them via the console in order for the enforcement team to investigate him. Calling out other gamers is not allowed on the forums.

I have reported him. I contacted support and they said to come here and fill out a forum post.

The best course of action you can take is to File a Complaint, Block
Communications with the Player, Mute the Player , and Submit a Player Review and Avoid the Player for each
player you feel is violating the Code of Conduct or Terms of Use. Just make sure not to send the player a message
after you block them as it will unblock this player. The enforcement team will eventually catch up to the player(s) and they will take the appropriate action against the individual(s). Just be patient and give it some time as the Enforcement Team wants to
ensure they are 100% certain in their investigation before taking any action. Also, it is against forum rules to post offending gamertags. Hope this clarifies what action you should take.

Use the report tool on your console, please. Don't report other players on the forums. Thank you.

Sorry, but support is incorrect when it comes to name changes. If you want to change your son's gamertag you will need to pay to change it. The enforcement team only issues forced changes when a gamertag violates the code of conduct. If your son is being
bullied at school, you would need to talk to his school about that. If he is being harrassed on Live, he would need to block communications with those people and file complaints on them via the console.