If this is the kid's first backpacking experience, don't get him a backpack - yet. Instead have him carry a daypack. Inside he can carry his own stuff: raingear, a warm layer, snack, bottle of water, headlamp, wistle, map and compass. You carry the rest.
Make it a short trip with a fun destination. Give him time to roam and explore. Give him some new gear of his very own: headlamp and multi-tool are good ones. That way he can be comfortable, have fun and you'll have to keep up with him rather then the other way round.
If he has fun the first couple of trips and begs to go on more, then is the time to go shopping for a backpack.