.22 definitely, single shot preferably, and one of the "chipmunk" models to fit him. He'll eventually outgrow it, but it'll be there for HIS children.
Rifles demand better technique (read:accuracy) to hit your target than a shotgun and promote good habits. Even the smallest shotgun kicks more than a .22 and may be too much for a slightly built boy.
Single shots are somewhat safer in that loading/reloading requires a specific procedure. A clip fed bolt action is certainly an option, but I'd stay away from a semi-auto for a first gun.
A bit of shopping should find you a used youth model for a very reasonable price, and short of using it for a crowbar, it's pretty hard to hurt a .22