I don't want to be friends with this kid
I am confused, why don't you want to be friends with the kid?
He is dirty.
It's obviously your right to choose who you want to be friends with. However, it's good to give people a chance and not to judge them based on their physical apperance.
I'm reporting him too to our principal.
If he's unsanitary or if he is actively harming someone, then you should report him. If he's minding his own business and not harming anyone then talking to the principal about him might be unnecessary. Instead, I think it would be nice to talk to him and gently and politely explain to him about the issue. Telling him to wear deodorant or bathe before school might help him fit in more.