My inclination would be to wait three or four years before giving him even a simple kit. My 7 year old son enjoys making things, but his attention span on any project is at best 30 mins – probably not bad for that age. At this age he’s still at the stage of building from Lego and cardboard boxes.
I bought him a cheap Chinese R/C toy boat for £25 to establish whether he enjoys sailing a model boat; so far he seems reasonably happy sailing it, although we took out the standard, crude R/C gear and fitted Futaba parts.
I think the old Keil Kraft balsa kits about 16 inches long are still available from the Hobbies Ltd catalogue, and are about as simple and cheap as you’ll get. I think they are about £10 with everything except the motor, and should be a good starting point for a slightly older child.