I'd reccommend starting by just putting him in the litterbox, to show him where it is. You might need to put more litterbox as around the house, so he doesn't have trouble finding one when he has to go. You can also try Cat Attract litter.
Make sure the litter box is low enough for the kitten to easily climb in and out of. If it isn't, add a sturdy step of some sort. Old phone books and Yellow Pages are great if you have any of those still lying around. Or a garden paver or two.