If you want a good rifle that will help in training you son the best rifle to get is the IZH-61. This rifle has an extendable stock that will grow with your son. It also has a 5 shot clip but uses side cocking spring piston power. It does not take much effort at all to cock the gun and is PERFECT for younger people. Im 26 and I have one, I love it to pieces and so will your son for sure. Here is a link to where you can see this fantastic gun. IZH-Baikal
http://www.baikalinc.ru/eng/prod/pguns/izh60_61/
and here is a place you can buy it from and the price is great considering what everyone else is asking for the gun
http://www.pyramydair.com/cgi-bin/model.pl?model_id=76