One other suggestion - see if you can get the kitten to watch the other cat pee in the box since kittens learn through observation.

Also, how tall is the litter box compared to the kitten? A small kitten might have trouble getting in and out of the box. We once made a "step" out of a book and cut out a part of the side of a litter box for a tiny kitten so that she could get in and out of the box on her own.

good luck.

(although I did once have a kitten that was perfectly healthy, and would pee everywhere. He was just always so excited to be doing something that I think he would forget to pee until it was too late He did outgrow this tendancy once he was a little older)