105 @ 5'8"? Get that kid some weight gainer! Kidding aside, do you have an expectation that he is still growing? I'm assuming this is why you're worried about buying a board that will likely be obsolete after this season. What about rentals at the mountain or from sports stores in town? This a no go because of the relative cost?
Not sure I would go much longer than 145, but I'm sure we have some skinny kids on here that can describe what they're riding at lighter weights.
*edit* Rocker is probably easier to learn on, but I learned on a camber deck and if anything it made me a better rider. You will most definitely want an appropriately sized camber board all that being said. The last thing your son needs is a stiff camber that is too long for him.
Last edited by Treegreen; 01-09-2013 at 04:02 PM.