I commend you for trying to train your new baby and for getting the other kitten a playmate. Put him in the box immediately after he finishes eating. Put him right back in if he jumps out. Wiping his tush with a wet rag is a signal to him to do his business. Stay with him until he does his business. Praise him highly when he is finished. Leave him in the bathroom during the night or when no human is home with him to cement the training. Keep the litterbox clean, many cats are fussy. A wire cage is also a good way to help him learn. He could have been a barn cat or an outdoor cat and never trained to a litterbox. Six weeks is really a little too early for a kitten to leave his cat momma. Eight weeks is reccommended.