I think the 28-gauge would be a great choice. The 20-gauge is a close second.
I own a Beretta 20-gauge & 28-gauge two barrel set over/under shotgun. This shotgun fits me very well. I have shot several thousand rounds with both the 20 & 28 gauges. There is very little difference in the felt recoil between the two gauges. The ammo for the 28-gauge is harder to find & more expensive than the 20-gauge. If you find a great buy on a 20-gauge I would buy it and pass on the 28-gauge. Either way you go you will have a starter gun for a young boy.