You need to call Res or go to the airport to book a ticket for a child under 15. This is to prevent circumventing the UM program. Res or the airport can confirm that he's actually accompanied. He can't be added to your PNR after you're ticketed. There aren't extra fees for booking through Res or at the airport... just costs the extra time.

The only elite benefit I'm aware of that he won't get is Comfort+ seating. He'll get your baggage allowance and boarding position.