Have you checked into options such as Gregory or Osprey? If you have an REI nearby that would be a good place to start. There are a lot of nice non hunting specific packs available and many of them have hip belts and shoulder straps that come in different sizes. That way you could just get a larger size hip belt and shoulder straps if needed as he grows. Might allow you to get a few more years use out of a pack before he needs a new one?
There have been some used Omni Suspension packs showing up for sale on the Kifaru forums. A Mollex/Express/Zulu and the beauty is that you can change out the belt sizes rather buy new packs as he grows. My Mollex would give him shoulder lift and 2300cui, and if necessary you can and back pocket, or long etc if he needs more space. Its hard to break Kifaru stuff as well which would be another plus for sure.
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.
Because young Scouts are growing so quickly what fit one year may not fit the next. When my youngest son crossed over from Webelos to Boy Scouts he was less than 100 pounds and under 5’ 6”. Now was a freshman he is 6’4” and about 160 pounds. Not only has his pack size changed but his ability to carry weight has increased dramatically. ALPS Mountaineering has a great program for selling packs to Scouts at a substantial discount through www.scoutdirect.com. They are entirely serviceable packs for Scouting activities. 5 boys from our troop used them at Philmont last year on a 75 mile 10 day trek with no issues. What we have done in my family is to purchase packs through this program as the boys grow and then as a gift for making Eagle Scout we buy them a Kifaru LongHunter. By the time they make Eagle they are big enough that a pack like that fits and they are able to carry heavier loads were that sort of pack make sense. It also means that I know they have a great pack to help me pack out elk. We donate the other packs to the troop so that boys that not have the ability to purchase a pack have something they can use while in Scouts.
osprey makes some great packs that can grow with a youth. I bought my son a adult osprey 65L pack a couple of years ago now. Shoulder straps can go from 16 to up to 20. They have sm, med and lg waistbelts.
Gregory is another great pack to look at, you may have to look at the wimmins packs to get one small enough.
Other than that it's pretty much like everything else with kids, your gonna buy multiple times as they grow. Don't even talk to me about how many pairs of shoes and boots I've bought lately.