I have to jump in and say that YOU probably want to carry most of his gear at first, and slowly transfer it over to him as he grows and ages. With my kids, I started with clothes, sleeping bag and pack. That was it. And that may have been a little too much to start with at 6 which is when I started my daughter. Definitely get a kid sized pack; a pack with too large a profile will catch the breeze easily and could toss the kid around on windy days. I ended up getting a nice little external frame for her, but it has been long since discontinued (a Kelty that ran about $50 back then). Check out REI for some reference points, but you may just want to get a cheap but comfortable day pack to start. Wait till he grows some and he will fit much better into what is out there.