My perspective...missing those first few days of school is really hard on a kid! Those are the times when teachers introduce class routines and rules and the kids get to know one another. Depending on where you live and how old your son is, it could matter a lot...or very little.
Does your school still give out class placements ahead of time? Here you wouldn't even know who your son's teacher is going to be until you get back, since they don't let you know what teachers your children have until the day before school starts.
That said, if it's the only time your husband can go, then go! I would discuss this with your son, however, and let him know you'll do your best to help him get into the school routine when you get home from the trip!