OK my suggestion is going to be one with an adjustable stock, like the one's with the AR collapsible one's
this way he can grow with it
as NO X box is going to be made for a kid that small, the problem is the distance from the grip to the trigger, even when you GET a shorter stock, that distance doesn't change, and will take time to learn and as he grows his grip will always be changing
I would also HIGHLY advice you make sure he learns to keep his fingers DOWN below the barrel of the X bow, so not to loose any fingers and that can mean, staying SUPER attentive when a deer in there and excitement is high
a X bow string when firing can take fingers right off!
I like seeing kids in the sport, but I am NOT a fan of kids this age doing the shooting, sorry don't mean to offend, just saying my thoughts, safety is a big concern to me for them, as excitement can make a kid do silly things , so be extra careful here please and best of luck to you!