I am not sure how old or big your son is but I assume he is a teen? I think the kids packs are for little kids.

My personal favorite pack is the Rogue, it isn't too bulky but can hold a decent amount of water with 70 oz and a few things in the pocket. I like it also because it isn't one that you will quickly outgrow when you move to longer rides. The classic just sacrifices some of the pockets but gives you the 70oz water capacity. I have only used the Camelbak brand myself so I can't really say if there is something better.

http://www.camelbak.com/Sports-Recre...012-Rogue.aspx

http://www.camelbak.com/Sports-Recre...2-Classic.aspx