The 20ga. would not kick him like a mule. I bought my first twelve gauge at ten. And used a 16 before that. If you tell him its gonna hurt him, it will. If you show him how to properly do it and show him how enjoyable it is, he would be just fine.

Now on to your question, youve been given some good info. And the price you found sounds pretty good. It'll also perform well on deer. I've seen them shut with .410 pumpkin balls, so good luck and keep us updated! Hope he gets one!
-JAke