TCB1-
This may well seem too simplistic, and this is coming from a parent, not a coach, but I would say that you allow each player's family one excused absence (1 game), the family will decide what is ABSOLUTELY necessary. You do not need to know the details, but they get just one. As this player's family has all of these months to contemplate their decision, perhaps they may come to a different decision or make an adjustment to their plans.
In any case, the ball is in their court. Let them know there will have to be make-ups for the other two games (whatever you decide), before the kid could return to his team status before the missed games. As a parent, I would weigh this against the planned trip as is, and perhaps after discussing it with son, make alternate arrangements to try to get back for whatever we could.
I will say this though, as a parent of a three sport athlete in his high school years, I don't really think the coaches appreciated what we as a family not only gave up because of sports, but sometimes they were downright ambivillant about the super human efforts our family made to accomadate each coach. Our kid was often on the road at 4 or 5:00am to play a game - multiple games (basketball)(tournaments) participate, jump in car sweaty, exhausted and starving, falling asleep as we raced to a double header where he was expected to jump out of the car and pitch a game, or if he did not pitch play both games every inning.
My point is - and this was prior to high school - the coaches only cared that he was there, did not care what we had gone through to get him there, or what family events we did not attend...we gave up a lot and looking back I think it was crazy. After getting to high school he was protected by whatever sport he was in at the time so it was better.
I was particulary disappointed after we had established ourselves as the ones who were always there, when son became ill with flu (very serious) when told that the only appt we could get with our doctor was during practice time, coach suggested that we go to emergency room after practice, not only was I ****ed that he thought so little of my son to suggest he postpone an urgent appt, but to assume that we could afford an emergency room visit!
I was not a happy camper, but guess what, son went to practice - sat in complete delerium - and the whole team ended up getting the flu - which led to greater problems for the coach.
Use common sense and be fair, if someone abuses the rules, dish out the consequences. If families have real needs, then give them their one pass, and let them figure out how they will deal with any more.