The thing with the light saboted bullets is they tend to have very light recoil as it is. Even with 90 grains, it's pretty much equivilant to the recoil of a round ball.
We're not talking about a 385gr conical like we used to.
I'd have him shoot a few at 75grains. Whether your kid knows it or not, I'd load 90 when hunting
If he's big enough to hunt, he should be fine with a full 90 grains with that bullet imo