I think you have figured out most of it already. Being positive, upselling the move, etc. are all important. But even then, he may still be very sad about leaving.
We moved 3 years ago when our little one was in preschool. Both boys didn't seem to have a problem at all. They started at a new school and preschool halfway through the year and everything was fine. However, our little guy told us a couple of times during the first few weeks that he hated the new house. I asked him why, but he couldn't say. All he knew was that he "just hated it". Anyway, this went on for the first couple of weeks. I always told him that it was ok to be sad about the old house, that I was a little bit sad too, but that he would get used to the new one eventually. I was so worried and heartbroken about it, especially since he was right, our old house had been way better because it was much bigger, newer and right by the park. I missed it so much too. Now, three years later he barley remembers our old house and basically this house is the only home he truly knows as his home.
One last thing. Take tons of pictures of everything preferably now when the house still is the way it always was. He will forget so soon. Having pictures of his old room etc. will help him remember his first home. As soon as the for sale sign goes up things will be different.