If you're getting decent patterns at 20 yards I say go for it. As long as he's shooting well it should be okay. Just be ready to pounce on the bird if need be. If he's up for shooting the 20, by all means let him try it. But I think you shouldn't push it on him since he's so young. It could scare him worse. If he does try the 20, go for 2 3/4" loads and add a couple layers of towel to his shoulder. That's what had to be done with me when I was 11 (little guy). Either way, make sure you get him out there and hopefully we'll be seeing pictures of a proud pop and a kid with an ear-to-ear smile. Best of luck.
-- B