flyguys... yep, go to a store and try them on. REI stores seem to have a pretty decent staff when it comes to pack fitting. Make sure your son gets a long torso pack. It took me several years to figure out that I needed a long torso pack. One other thing to remember is that lighter isn't always better. Some companies shave weight off of the frame & support structures to cut weight, because light weight sells & is the only spec a lot of people look at.
I can't really reccomend a brand. I've had most of them at one time or another and they all have their good eggs & bad ones.