Just make sure the 20g has a good recoil pad on it, and avoid the heavy shells. Id also recommend ear plugs. Alot of people have a hard time hittin dove so make sure you miss a few too, so not to make him feel bad. When you take a kid hunting, make sure its about HIM hunting and learning, not about a bird count.
Theres still time to take him out for some clay pigeons also. Great way for him to learn a shotgun, and to figure out what'll work for him.