Some of you are missing a HUGE point: this isn't the kid's decision. It isn't like the kid is planning the vacation. It's his parents. You are punishing the kid for the actions of the parents. You are considering taking away a privilege the student values based upon the decisions of the parents who didn't do not value the privilege in the same way. This is not right in my book. If the kid just decided to not show up for no reason other than they didn't want to show up, that's one thing. The kid can't help that his family is going on vacation at that time. COVID-19 or not, do you know when the most popular time to take vacations in America is? Late July/early August. It has always been like that. Someone asked if they would go on vacation during this time if they were on the football team? Probably not, but again that is a determined value by the parents, not the kid. And, what if they did? Coach would probably still keep them on the team and if the kid was good enough, even start. I correlate this directly to the kid being sick. In both instances the kid can't help it. They are being forced into this situation by something or someone beyond their control. I know it is tough to understand this but band is not the center of these kids or families world. If anything, this current situation has showed us all that. Band is our life, and yes it does hurt when others don't feel the same way we do. We take it personal because we've invested our lives into this.
If it were me, I'd let the kid still apply for a leadership position and take into account their past record and accomplishments. If they've done enough to earn a leadership position, great. If not, don't let it be because of something beyond their control. You can always remove the student from the leadership position later if dedication and attendance becomes a problem (I'm sure you have a clause or provision for that in your handbook).