K.S.
My son was 3 when we moved into our current house & I was nervous about the transition. We really "talked" up the new place & how special his new bedroom was going to be. Since we were building, we were able to visit often (I don't know if that's a possibility for you) but maybe even driving by or finding a nearby playground would spark some excitement for the new place. Together we picked out new nightlights and other accessories for his new room which seemed to help. Maybe put together a memory photo book at your current house with photos of your son's favorite "things" (outside playing with friends, in his room, etc.)
Best of luck :)