First step is to ask him if he could only play 1 sport in HS, what would it be?
He may pick FB. In some schools, FB is a huge thing and hockey is not. High school sports have a social lever that is important to kids.
And if he picks FB, out him up for adoption this weekend.
I'd say no chance any kid could swing both. Might burn him out on sports entirely. One or the other.
Do it! Alot of us and our kids were/are 3 sport athletes in high school (Football, Hockey, Baseball/Lacrosse)..Id rather have a tired kid than a kid with time to get into the crap available to these kids today. You will have to deal with conflicts but its life and a good coach will understand that.
Is he also planning on playing hockey in HS?
I can't imagine a U-14 FS coach on a National bound team being very receptive to having a player who has a priority in front of hockey and if he's playing HS football that HAS to be the priority. The advice on stepping down a level for a team that is simply happy to have his talent is good from a "peace on earth" perspective but isn't going to help much for his hockey development & it probably won't do much to get him ready for HS tryouts if he's planning on going that route.
As was pointed out, there is also an important social aspect to this equation as well.
Certainly let him play football if that's what he wants. If it was my kid, I wouldn't go to a U-14 team, just play football during the fall. However if he wants to tryout for hockey I would have him on the ice on weekends doing skills and maybe looking for a spot as an alternate on a team. If the HS tryouts don't work out, chances are you'll be able to find a spot on a U-14 team after the conclusion of HS tryouts.