Found a teeny kitten

He has teeth, his eyes are open. So I’m assuming he can have wet kitten food. The spoon is almost bigger than him and he’s extremely malnourished and skinny (don’t let the fluff fool you). He’s eating. Taking him to the vet tomorrow. I’m just wondering if he’s too young or small of a litter box, or do I was to stimulate his butt for him to poop and pee? Thanks