I am looking to buy a backpack for my son who is in Boy Scouts. He will be doing some high adventure outings over the next several years, so I would like something that may be able to grow with him. I'm not sure what size (liters) pack we should be looking at. He is 14 years old and is 5' 8.5" tall and growing. I don't want to break the bank either. Any help with pack size and adjust-ability is greatly appreciated. The last time I bought a pack was when I was in Boy Scouts and it was an external frame.