The Umarex Heckler and Koch P30 CO2 pistol shoots both pellets (from an 8 shot magazine like other Umarex guns) but can also fire 15 BB's too! It looks the business, has a rifled barrel, is double or single action and the trigger is much better than the other Umarex guns, should easily be light enough for a child! When using double action, the trigger travel is quite long as the first part has to cycle the magazine, however, if you manually cock the gun for each shot you have already cycled the mag and the trigger is single action and has a much shorter reach and travel!
Rear sight is adjustable for windage only so elevation requires some guesswork but once he becomes familiar with the gun he'll know how far to aim off for the range...
If I was to buy another Umarex pistol, this would be the one!
Others to consider are a Crosman 2240, which is a good starter gun but is single shot, not gonna be as much fun as the H&K...