go with a .22 single shot rifle, there are lots of them out there. Avoid the scope, no need in giving him something that will ruin everything he has been taught at camp about proper sight picture and sight alignment. Focus on getting the fundemental skills of using iron sights first. Skip the .410 and go with a 20 ga. He will enjoy it more and so will you. His chance of hitting his target with a 20 ga is increased over the 410. Really a 410 is an expert gun, with the 20 you can focus on the general techniques of point and shoot.
Glad you get the opportunity to enjoy the outoors, and open up the opportunities of shooting sports with your son. With proper safety procedures practiced you will never regret the times you spend together with him.
GySgt USMC (ret)
Boy Scout Shooting Sports Director
NRA Instructor and Range Safety Officer.