See jenlukenbach's posts about Lucas in Cat General. She has had really good luck with canned pumpkin. And some Metamucil, but she started with just the pumpkin.

Are there any bits of stuff around that your kitten might swallow?? That's the only other thing I can think of!

Try the pumpkin and good luck - they are still young enough that they might like it, and form a good habit.

Catty1