The problem isn't the rifle, or your grandson, its the time factor. Certainly your grandson will be able to learn to handle and shoot well that 6mm Rem. BUT, is it time for him to begin NOW?
Too much gun, too soon has ruined many people on shooting. And its not always just the recoil. Size, weight, and fit of the gun, plus blast and recoil, all go together.
I have several model 600s, in calibers from .222 to .350Mag. I started my deer hunting at 16 with one in .308Win. And yes, it kicks!
I think the lad could certainly handle the 6mm at 14. At 12, most likely. At 10, possibly, if well coached. But at 8? I think he might be willing, but the gun is just too big, too soon.
Now, that's just an average, and your lad might be the exception, and take to it like a duck to water. But, if he doesn't, you can do more harm than good. I'd give him some more time, before bringing out the bigger stuff. 6mm is light to me, for a deer round (works fine, but on the light end for caliber), and sadly, I can't remember what its like to be 8, anymore. But I think to him, its still pretty big.
Just my opinon, and worth what you paid for it.