Hi D.,
I have not read the book suggested by another memeber... but it certainly sounds like a great place to start.
In addition, you may want to help him find a way to express his grief. Maybe through coloring pictures or writing stories about the kitten. Often times children have a difficult time knowing how to express all the emotions they have inside and get frustrated. Offering him tools to help him move through the process will be beneficial.
Also, keep in mind this is still pretty fresh. He may only be 6, but he is still grieving. His world is fairly small right now.... there was probably a lot of love attached to that kitten. Think about how you might feel if a close friend died ... I'm guessing it will be similar to how he is feeling right now.
I'm guessing the wanting of another cat will pass eventually...but it is obviously part of his grieving process right now. Helping him express his emotions in another way may take the emphasis off wanting another cat.
You can also remind him that although your other cat and dog can not replace the kitty, they still need love and attention.
Sorry for your loss.