I'm going to take a different approach with this than the advice you've been given. I have many dogs and several cats. The dogs are not allowed to chase the cats, EVER. Showing too much interest in a cat gets the dog booted outside. The house is the cats' territory.

When I introduce a new dog, they are always kept on leash. The cats always have an escape route and at least one section of the house is blocked off from the dogs. Even the dogs who are good with the cats are never left alone with them--ever. They are always supervised when they are together.

9 weeks ago my dogs killed one of my cats when she got outside. I don't believe they meant to. I think they were playing with her--chasing her like they do squirrels. A dog, even a puppy, can kill a cat extremely quickly, easily and accidently. Trust me, you don't want to go through this!

Your puppy needs to be firmly and consistently told NO when he chases a cat. He should be kept leashed or crated until he learns not to chase.