Kitten wont use the litterbox

I put my kitten in the pet carrier that I had just cleaned out for him, I put him and the litter box in there, nothing else, went back to let him out 10 minutes later to feed him. And there's pee and poop everywhere... I am so lost at what to do. I've done everything I can do to potty train him! Someone please help! I love him to death but I have a 7 month old who doesn't need to be exposed to the smell of cat mess. Since he potty's where ever he wants, If he can't potty in the litter box,idk if we can keep him!!