Why 20 gauge? Is it weight or recoil? There are so many more options in 12 gauge if weight is not a big deal. If you reload you could easily drop down to 3/4 ounces so recoil would be almost nothing. Then as he grew heavier loads could be used. I often see beginners started out with a .410, a caliber I love but it is almost useless to a beginner trying to hit flying targets.

My father started all of his boys out with a 20 single shot. As soon as we could handle that we were moved to other guns. My Christmas gift at age 12 was a Winchester Model 12 Skeet gun in 28 gauge with a split case of one ounce Winchester 6s and 7 1/2s. With that I hunted everything. What a gift that was.