Bring 2 extra sets of clothes... One for the trip home, and one the youngster can change into mid-hunt if needed. Also, food and some hot choclate. If you blind hunting, a place for him to sit would be good. Also, he's a KID, he will not be able to sit still for anywhere near as long as an adult. When things are not happening, let him move around, and be a part of setting decoys and moving them if you need too.
Most of all, make it fun for him. Oh, and ear muffs or plugs.
Thats my .02 worth.