We've moved tons. More times that I can count. Our last move was the end of Nov. and we are still in temporary housing.
It sounds like you are off to a good start. Show him a map of your trip. If some of your things will be going into storage for a while, explain that, too. Let him help decide what will stay with you and what will go to storage. If you have pets talk a great deal about what is happening to the pets. This is a big issue for young kids.
We said good bye to our last house and walk all around the yard saying good bye to the trees, rocks etc. It made me feel very sad, but my kids (who were 3 and 5 at the time) really loved it. I had a special moving day present for each of them in the car. Obviously, something they could play with in the car! Unwrapping it in the car was very exciting and fun.
Since you are moving to where you have family, have them check out what is available for kids there -- story time at the library, classes through parks and rec., children's museum, etc. Talk about this stuff a great deal with your son. When we moved from Canada last fall it was already cold, but I had told my kids that it would be warm enough to go to the zoo here in southern Arizona when we got here. It gave them something concrete to look foward to. (And we went the zoo when we had been here less than a week).
If there are issues of concern or uncertainty, don't talk about them infront of you DS. This is scary for kids, and it is easier for them if we appear to have it all under control.
One mistake that I made with an earilier move was making a big deal about my DD (who was 2 1/2) saying good bye to her friends and making little photo ablum of them for her to keep. Looking back, this just made it harder for her to let go those kids. With this last move (my kids were 3 and 5) I said as little as I could about their friends and I only talk about them now when they bring them up. We saw their friend one last time and said good bye, but I keep my emotions out of it and it a fun time instead of a sad time.
I haven't had good luck finding kids' books about about cross country moves. Books and that say over and over than mommy will still take you over to play with your favorite buddy just aren't helpful to child moving thousands of miles.
Both of my kids have gone through some regression in every move. What ever the last new still is that they have mastered, they seem to loose ( like potty training, sleeping through the night, etc.)
Save some of the big boxes after your move and make a puppet threater or play house (or both!)
Linda -- who had one child in Wichita, one in Little Rock (where we lived in an apartment and then a house), lived for a while in sunny Tucson (3 months in an apartment and then 11 months in a house), headed up to Montreal (horrid little town house right next the airport), lived in the country side in Ontario (a great house on the water), and then came back to Tucson -- in 5 short years! I can no longer remember my phone number.