I wouldnt get a kit, get him something he can go shoot that day. Everyone will probably say what a great time it would be to build a gun like that with your dad, and yeah, it would be. But, when you were that age, which you have rather gotten? I would have rather gotten something I could shoot right away.
T/C doesnt make traditional muzzle loaders anymore, so if thats the route you are looking I would look at a Lyman. Tons of choices if he doesnt want a traditional rifle. Regardless of the route you go, teach him to clean it right after shooting. Even Blackhorn 209 will corrode the barrel if you wait too long.
