I think I'd really want stress to him to not be one of the "cool" boys. That would be the main thing I'd want stressed to him. I'd tell him that in a few years, everything will be one hundred times better for you than any of your classmates if you just keep being yourself, as weird as people may tell you you are. That everything he's into right now is fine, despite what anyone says to the contrary. I'd really stress to him to keep being friends with those nice girls in his class, and to not be friends with the stupid idiotic boys. Everything about him is fine, and to not let other people tell him otherwise. I'd really tell him that God loves him, too, and to keep being good, despite all the people around him he sees prospering from being evil. I'd tell him to avoid being in the presence of wicked people at all costs, even if you have no friends because of it, and men ridicule you for it.
I'd tell him if he doesn't take this advice, his future will be very grim, and he'll see jail bars, blood, and everything will be terrible for him, but that God loves him so much he'll fix things regardless of how much he screws up.
Oddly, 10 years old, which I think was when I was in 4th grade, was the turning point for me, I wish I had someone say this all to me at 10. I remember I saw a kid who figure skated at the skating rink, he seemed so much like me at that age, I told him to keep figure skating and to not let other people making fun of him stop him from it, and that when he grows up, life will be great for him.