100% be honest with the coach. If he has a problem with it then too bad. What are you going to lie, and then have your son come back with a tan in the middle of march from being in the warm sun of Costa Rica for a week? Not to mention there is no doubt your son will want to tell his friends about the trip and ultimately the coach will find out.
This is an absolute no brainer to just tell the coach the kid is going on a school trip. I also would tell him sooner than later that your son is planning to go on this trip. This way if the coach acts like a jerk the rest of the season you have plenty of time to find another team and aren't scrambling after playoffs/before tryouts to go somewhere else.
OP last time here. good call. I'll wait for a few weeks though because 1) the trip still may not happen and 2) in case if the coach is reading this board, I want his memory to fade a little before getting hit with a "so hey my kid needs to go on a class trip to Costa Rica"