Moving is hard for anyone...but a teenagers world ends at their nose...so it's especially hard for them. I moved A LOT growing up. I went to 5 different H.S. by my sophmore year! yikes. It was tough...but in the end I learned so much. I can easily make new friends because I had to. I have friends all over the country. And while it was hard to leave friends and family behind...in a month or so I was excited about the new people I was meeting and the new area. Maybe let him pick out his own room in the new house, decorate it how he wants, tell him it's a new beginning so he can reinvent himself if he'd like. I was sort of shy at one school and decided on my next move I'd be super outgoing and just see what happens. I figured I'd wouldn't be there long so who cares? But I wound up staying for a few years and I made great friends and had a great time.
All in all, he won't be happy with you guys for a period of time. But he is your son now and you make the decisions. He doesn't have a choice to stay or go...he's going. Just try to make it as nice for him as possible. Like you said, he can go home to visit on breaks....but this is not his decision. He only has 2 yrs left of school and sometimes it's nice being the new kid. :)
Good luck!