I'm not trying to be an old schooler here. However, I'd be having him work on proper form with no props.
I went off to hunt turkeys just before #3 son turned 4. My wife called me that evening and told me he'd grabbed one of my decoys I'd left behind and his toy SXS shotgun and was out in the back yard calling. After that, I always took him with me.
My point is, once you've shown him proper form, he should be able to practice it and get it right.
Here's mine about age 14.