I attended a BYU-Utah game at the Huntsman Center a few years back, decked from head to toe in BYU gear (back then, I thought my matching BYU sweat suit was cool haha). I had a great time. My experience with Utah fans there was very positive.
In my opinion, the Cougarboard obsession with Utah fans is a little overblown. Their fans are a lot better than they get credit for. That's especially true of their basketball program (doesn't seem to attract some demographics that football does), and it's even more true when they are having a down year. It will be a good atmosphere for you and your son.