The only problem with him enjoying it so much (like fishing) is he asks me every day (sometimes 8 or 10 times) if we can go shoot again. Now, he's asking me about the 12 ga shotgun. I have clay targets and a target thrower, but I'm not sure at just over 100 lb whether he'll be able to take the 12 ga kick. I may start him off with the 20 ga or see about getting a .410. I think .410s are great starter shotguns. I also see a gun safe in my future. $$$
I got my wife a 20 gauge thinking the kick would be lighter than a 12 gauge but it doesn't seem to be for her. The 20 gauge is a little lighter in weight but that makes the kick worse.
There are not near as many varieties of shot for the 20 gauge available as the 12 either and the price is about the same per shot for less material.
I had a .410 from Monkey Wards when I was a kid. Great gun for young shooters.
For your grandson a shorter stock might be important depending on how big he is.
Ron