It's perfectly normal to freak yourself out before heading to USNA, or any of the service academies. The bottom line is that it's not that bad. If he's a self-reliant kid, and it sounds like he is if he did the whole application process/got interested in a SA on his own, he'll be fine.
Sure, plebe summer/year are hard, but it just gets harder. It's why you hear upperclass talk about how plebe summer is a joke (and why it's so funny to hear/see what plebes freak out about over plebe summer).
The good news is, he'll be ready when it does get tougher. The stuff you tackle early on pushes you and gets you adjusted so you can take on the stuff later on that's more challenging. For example, I would not have done nearly as well at, say, Leatherneck if I'd gone straight out of high school or plebe year as I did after three years at the Academy.
That applies to all of the Academy's "mission areas," which are moral, mental, and physical development. You start out and the expectations are relatively low, but by meeting (and hopefully exceeding!) them, you get yourself ready to do the hard stuff.
I can't speak to the missing people element as much. I'm the youngest and my family lives on the eastern seaboard, so I both had already dealt with the not-seeing-siblings-all-the-time thing and didn't have to deal as much with not seeing my family because I could go home for Thanksgiving, etc., relatively easily. I have never been homesick, but people are different.
The good news here is there will be an almost instantaneous support network for him, both in his company and out. His classmates in company will look out for him: if he can't make it back for Thanksgiving, someone will offer to let him stay at their house, pretty much without exception. If he has nowhere to go on a long weekend, he will find something to do, whether it's going to his sponsor's house, or spending time with someone from his company or an ECA.
The bottom line is at this point, all he can do to get ready is work out, put out in school, and not be a dirtbag. If he does that now, then it'll all sort itself out once he gets here. Every year, 1000+ people come here and figure it out. They're not supermen, they're normal kids who put in some serious effort and make it work. If he wants it, it's on him to do the same.