He keeps asking if the new house has his room in it.

You've shown him photos. You've told him his bed is coming. He nods, and then twenty minutes later he asks again, in the same worried voice, whether his room is coming too. He is not being difficult. He has no idea what “moving” means — for all he knows the house comes apart and everything in it is up for grabs, including him.

What actually helps

• Be literal about what travels. His bed, his blanket, the lamp, the box with the dinosaurs, you, the cat. Name the actual objects. Abstractions like “all your things” don't land; “your blue lamp goes in the van and comes out in your new room” does.
• Say what happens to the old house, plainly. Another family will live there. It doesn't disappear and it isn't broken. Children fill silence with something worse than the truth.
• Show him the new room empty, then with his stuff in it — photos are fine if you can't visit. The empty version is the one that scares him, so don't leave it as the last image he has.
• Give him one box he controls. He packs it, he tapes it, it rides in the car, not the van. When everything else is out of his hands, one box that is entirely his does a surprising amount of work.
• Unpack his room first, before the kitchen, before anything sensible. One familiar room on the first night is worth more than a working coffee machine.
• Expect it to land late. The wobble often arrives two or three weeks after the move, once the novelty wears off. That's normal, and it isn't a sign the move was wrong.

Why a book helps here, when talking doesn't

He's asking the same question repeatedly because he can't hold the answer — it's too abstract and there's nothing to look at. A book turns the answer into something he can pick up and check for himself, as many times as he needs, without having to catch an adult who's busy taping boxes.

A book about his house. Not a house.

His name, his room, the lamp, the bed, the street, the van, the new room with his things in it at the end.

Written the way that works: the old house is allowed to be missed, the ending is the new room with his stuff already in it, and nobody pretends the whole thing is an exciting adventure if he doesn't think it is.

Or the whole book in outline, for him to color.

You choose how it's drawn: painted and full color to read tonight, or the same story in plain outline for him to color himself.

Coloring the new room before he's slept in it is a way of getting there first.