Let's Go Pikachu would be great for a kid of any age, it's pretty chill, like I didn't enjoy the throwing mechanic, but he'll probably dig it and its otherwise a good nostalgia trip being a remake of the original games. I'd certainly recommend it as a starter game along with New Pokemon Snap. Then probably Sword/Shield as that one is fine enough (I think it's a bit crap, but a young kid won't care) and then Arceus. I'd never recommend Scarlet/Violet on Switch personally due to the reasons I've listed already, though if your kid likes the other ones, there might be no other option and they may or may not care, I don't know lol