I'm shocked that it took 6 1/2 hrs to get that response. /\ If I were in your shoes it would depend on whether this is just until he's strong enough to pull a bow back or whether he will be sticking with a crossbow for a while. If he's gonna use a xbow for a while I'd look at ravin's or excalibur. If its just to get him through until he can use draw a bow I'd buy a wicked ridge and be done with it. *edit* In my experience "light" and crossbow don't fit in the same sentence.
I'm a big fan of Excalibur, but they're sort of the opposite of what you're wanting. Simple, relatively bombproof, but the draw weights are pretty stout, so definitely not kid friendly.
If I were looking to replace my Xbow with something a kid could handle, I'd look at Killer Instinct, and probably Mission. I think Mission has one that's intended for women, a bit easier draw and probably smaller/lighter geometry so that might work well for your son. A hand crank is going to be much easier to use (strength wise) than rope cocker, though it's pretty slow/tendious when practicing.
Crossbow's are actually a lot more fun than I would have thought. I really enjoy shooting a vertical bow, but shooting a crossbow is its own type of fun. The accuracy and power is pretty impressive as well, though you're still limited by the arrow, so IMO, they're far closer to a bow in terms of hunting style/limitations (especially modern compound) than a firearm.