Why wont our kitten use the litter box?

5-6wks. is too young to leave momma cat. Ideally he should have stayed with his mom until 8 wks. or more. He would have been better adjusted had he more time to learn from Mom & litter mates. But since he didn't you are just going to have to be patient and vigilant. I would confine him to 1 room and when he wakes up, eats, or drinks, put him in the litter box. Also if he starts looking in corners or scratching floor or going in circles~ to the litter box he goes. But be careful not to be rough or startle him, you don't want him to ociate the L.B. with a negative.Good luck and remember ~ be patient and consistent.