My daughter dances -- not competitive dance team but ballet. It is a very serious program and the kids are expected to be at all classes and all rehearsals; however, the studio directors have the sense not to schedule something like a rehearsal on a holiday. There are classes on some holidays (Columbus Day, for instance) but they know better than to put them on a day like Labor Day -- when traditionally MANY families travel, at least around here. If this were in our area, a ton of kids would have missed that practice your son missed. Would coach bust them all down to another team?
I question whether this coach was using the Labor Day practice as either (1) a crude way to test players' commitment: "If they don't come today they aren't committed enough!" or (2) a way to make up required practice time he should have had them doing earlier and HE failed to do.
Either way, I would keep a close eye on whether this coach is going to be a "Football is your life now" type of coach or if he recognizes that families have lives. Yes, we have given up many things for our child's main activity (which takes as many hours a week as most sports and competitive dance, believe me). We have not done trips, she has missed activities at Girl Scouts and even at school, BUT we also have had times when -- with advance notice from us to the studio -- she has missed dance events, even rehearsals (rarely) for something very important; it has to be truly important, like a graded participation in something outside the school day, for her to miss a rehearsal - but we'll do it.
What really would trouble me in your shoes is this: Was the "youth event" a church-related event? That's what that term would mean to me. If your son is expected to put his own religious growth behind football -- that's a red flag. For reasons of safety and preparedness, of course the coach can deny your son a slot in the next game if he misses a practice, but busting him down to the lower team for missing a single practice -- IF you notified the coach in advance -- smacks of pure punishment for not giving football top priority over church and family.
I see a lot of arrogant sports coaches and dance teachers; fortunately not at our studio. There is an expectation now that when a kid, especially a teenager, is involved in an activity the kid must treat it like a pre-professional and put it ahead of everything else. Sorry, but religion and graded schoolwork come first. The vast majority of the time, there usually won't be a conflict. But when there is, especially when you have tried your best to work things out so a child can be at the activity to which they're committed -- it's pathetic to see adult coaches who treat kids like dirt for daring to do anything other than the coach's chosen sport or activity.