In my personal opinion, he should enjoy the trip. He can prepare before he leaves. Depending on the length of the trip it should not be a big factor, he can still do some of the exercises daily while on the trip. It may be one of those things he can look back on fondly during the harder days of BCT when he is trying to make it meal to meal or hour to hour(there are those kids who, that is the only way they make it through). They need to enjoy the time before just a much as staying fit. Their college experience will be much different than their peers, why start making it harder before they get there? I know at least one of my kids and probably both would have resented me terribly if this was them.
It is also tough adjusting to altitude, however neither of my kids went early enough to make a difference in the adjustment. It truly takes them 3-6 weeks to adjust. The Academy knows this! Not going to lie, it was hard on them (coming from sea level) but both made it through BCT with a sense of accomplishment.