J.W.
Have you just told him things will return to normal in just a little bit? Kids that age have no concept of time so they think things will continue until they stop.
When we built our house my older two were 2 and 4, they were out there with me at least three days a week looking at the house. They picked out their bedrooms when they were just framed, I asked what color they would like the hall bath since they would use that. We dreamed about the house together.
Although you can't go back in time and do that you may be able to reference times he did see the house so that he understands the process. Because of including them in the process they just both set to setting up their part of the world as if that was the most normal thing to do.
I guess I am saying you need to help him find his sense of control again, that is all he has lost.