I dont know what you plan on spending, but i have a 30in waist and i can suck the kifaru belt down very tight. Also the suspension is adjustable, allowing you to make it bigger as he grows, and also change out belts if that is needed. The shoulder straps also work very well for smaller guys, there seems to be more wiggle room in them rather than just having them sewn solid. For the record, all the above info is based off the timberline pack.
A kifaru pack would really be a 1 time purchase thing if he plans on using it year after year or what not. It will fit no matter what as you can adjust the pack alot! Even if you wanted to use it, all it would take is some easy adjustment and possibly a bigger waist belt if you are any bigger.
That being said it is expensive and that may be an issue if its a 1 time use pack, or will get lightly used.