Be sure that the UM policy does not require that the minor be 15 to fly totally on their own. Most don't.
If this is int'l, you can't fudge on the birthdate because of the passport.
Be aware that if there are any connections, and if there are any problems, he is too young to get a hotel room on his own. Not a problem if it's a non-stop. If not, if you know anyone in the connection city, you might want to contact them, just in case.