Accuracy isn't the problem with Encores. I have barrels in 270/300wm and 45/70. They all shot sub inch groups with hunting bullets when I do my part. BUT they are light rifles and not easy to shot well when the recoil gets heavy. Your caliber suggestions are max for a 12y old IMO.

Stay away from the belted cartridges, extraction problems! Standard cases work fine, rimmed ones even better, just twist the rifle to clear the chamber.

Every 12y old boy should have their own rifle, and the Encore is a safe and logical one to start out with.

I was leaning most towards the .257 Rbts, but maybe even the .250 Savage. Ammo selection is somewhat better with the Roberts. At this point, the largest firearm that he has shot is a 20 gauge at bluerock. We may have to ease into it.