The kid's Dad was an all-conference player at BYU and coached at BYU for several years. He's probably been attending BYU games since a kid and has probably been inundated with all things related to the Y from a young age. I could be wrong, but I assume he's probably been a hardcore Y fan his whole life.
If those assumptions are true, just seems really odd to commit to your despised rival. I would guess there is more to the story if that happens.