I second ChoatieMom–play it by ear. Some boarding schools are intense experiences. At first, they often wanted to eat a home-cooked meal, catch up on sleep, meet friends and do laundry. If your son is thriving, he may want to visit new friends–or invite some friends home.
There are also school travel opportunities planned for breaks. Community service trips may take a week or so from the time allotted for vacation. Sports teams often use some of the spring break time for training in Florida. Music performance groups may travel within the country or tour internationally. There are also lots of camps and/or internships which take place in the summer.
So I wouldn’t be putting down a non-refundable deposit for two weeks in an expensive location, until you know what your son would like to do.
If you live in New York, you may find yourself visited by your son’s school friends, as they deal with travel home. (And it’s a good sign if they like your son enough to ask.) In our experience, August is the best time to plan a family get away. Check when your son’s expected back at school, though. The calendar for the next school year usually goes up around Christmas or so.