I think airsoft is a bad idea if your training a new/young shooter to shoot a real gun. He said he has been shot with one so he won't get the idea. As posted above, I would go with the single action, break open pellet guns. They make them in rifle and pistol.
I think its great your taking the time to do this with your grandson. I came from a gun free home and didn't start shooting til I was 18. Didn't get my first gun til I was 21.
Thanks for sharing the sport.
Novaking