That is great that you have a high school student wanting to help mentor the team! I think that is one of the great things about all of the FIRST programs. However, lead coaches have to be 18 and older, with the new Youth Protection Program, and have to undergo background checks. Every team has to have two adult lead coaches now. You can find the handbook here:
As far as I know, he can still be a mentor, and you can write in his name as a mentor on the Team Info Sheet when you go to tournaments, but he can't be one of your two lead coaches.