As a real estate consultant, I encounter this often and actually encourage the parents to talk with the kids early in the process. Let him decide which house to live in if possible. A house becomes so much more than 4 walls for children, it is usually the center of their world. A comforting place and the thought of moving can really disrupt that leaving something strange in its place. The things that were once so familiar are now different almost like living is someone else's space.
Talk to your son about the move in as positive a way as you can. Ask what he likes about the current house and neighborhood and then stress and show that the new house will have all that and more. Don't just promise to let him decorate his room. Take him to stores to start picking out what he wants or better yet get him a gift card so he feels he is in total control. If possible take him to the new neighborhood to play so he can possibly meet some new friends on the playground. Once school starts you might be able to take him there to visit.
I hope this helps and best of luck!