If he feels that way, he should do what he feels is right. I'm tired saying this on here and elsewhere. Underage football will only go so far to preparing you for Senior. For years now, we have looked at young lads, under developed and not properly tested, and hailed them as the next big thing. I'm guilty of it myself. It rarely happens. If you're good enough to be playing with the Seniors, enjoy it and learn as much as you can.
If you're only on the panel with the Seniors, common sense dictates that you should be allowed to play with the 20s, if only to get game time. A bit like the squad players are allowed to do in the rugby