what kind of 20ga
If the 20ga the kid is struggling with is a single barrel, I'm not surprised he's having trouble. I tried to set one up for bamaboy as a youngster for spring gobblers........too light. Kicked (it seemed anyhow) as much as any 12 ga.
Even some of the 20 ga pumps are pretty light. If you can swing it, a 20 ga gas gun, with a good pad, should be much more shootable than a light single, double, or pump 20. Ought to be some out there somewhere, used and so on.