I've seen CAPT MJ post this advice before, and it probably the best single point in these threads. If DS truly wants to leave, encourage him to be running toward something, instead of away from USNA. Something drew him to USNA in the first place, ask him what has changed ?
This issue comes up frequently, and if someone did an analysis, they would likely find that Thanksgiving (the first time to stop, think and ponder, particularly if he was left alone in the Hall) and post Christmas (as the Midshipman hit the Dark Ages) are probably the most common times.
The Plebes are coming up to one of the best times to be a Midshipman (BEAT ARMY!) and one of the worst (first experience with College level finals). You should encourage your son to put his head down, finish this first semester hard, and if necessary talk to the Chaplain and/or Chain of Command before he leaves for Christmas break. Don't make any rash decisions, or even consider throwing life lines like holding off buying post Christmas return tickets.
Finally, while I understand the natural instinct to want to go visit, I would discourage it. First, he is going to be very busy through finals. Second, I am sure that some of the parents here will disagree, but I think that it is unlikely that a parent has the insight into life a Service Academies to really understand what a Midshipman in this situation is going through. The chaplains and chain of command know, and understand, and are better prepared to help him make a decision that he will be able to live with for the rest of his life.