We moved across the country when my oldest was three. It was a tough transition for him - my most important advice is to be patient and let him adjust at his own speed. Some things that really helped him: picking out a color for the walls of his room, having his room (with all his stuff) set up as completely as possible first thing (we were moving near my parents, so we stayed at their house until we had unpacked enough for our house to be livable - this was ideal.) When we were packing up the old house, I made sure that he was with his favorite babysitter - on moving day as well, so that he was with someone he trusted and loved, but also so that he could watch when he wanted to, and when he needed to step away and do something else, he could do that, too. I made a photo collage (in one of those multi-photo frames) of the old house and pictures of him at the park and by the river and with his old friends and important grown-ups - to help him remember, and to help him know that those places and people remain important, even if we don't see them often anymore.
The "Berenstain bears moving day" was a good book, but one that became a favorite was "Big Ernie's New Home" - about a boy and his cat who move from San Francisco to New Mexico - the boy is happy enough about the move, but the cat is not so sure. It spends a lot of time describing favorite things about the old home (smells, sounds, colors, etc) and then the discovery of new favorite things in the new home.
http://www.amazon.com/Big-Ernies-New-Home-Children/dp/159...
Good luck, and remember that it takes a full year in a new place to really feel settled and at home - for you and for your son. Be patient and gentle with yourself and with him. Best of luck.