How old is he? I'd say 'regular' blasters (the vast majority) are suitable for ages 7+ because they require some arm strength to prime. However, I recently witnessed a kid around that age fail to pull back the slide of an X-Shot Xcess so I don't know.
For really young kids, Little Tikes has the Mighty Blasters range of bean bag shooters
For teens and above, there's Nerf Rival, X-Shot Chaos and the Adventure Force Tactical Strike ball blasters
Dart Zone Pro/Adventure Force Pro blasters are the most powerful dart blasters you can find on (North American) shelves but they're probably overkill unless he plays in organised games where modified blasters are allowed