Hunt out of a ground blind, do what everyone else suggested keeping it short and to keep him occupied and when that fails a Nintendo DS will entertain a kid for hours which will keep him quiet and focused on something other than his butt falling asleep or being bored. My friends daughter has been shooting deer since she was 5 and I think she is up to 5 deer already at the age of 7. She is already holding out on small buck and waiting for the bigger ones. I think she will be impossible to please when she gets older and it will lose it's fun for her.
Another huge help is make sure the kid see's deer when they are out with you, something that is becoming extremely difficult to do here in Pa. Food plots on unpressured private land are the way to go for a youngster just getting into hunting. Sitting in the cold and not seeing anything isn't something that makes a kid want to do it again.