Unless you actually stop Harley chasing candy, nothing will change, if you always do what you've alway done, you will always get what you always got.

How you stop him really depends on how strong the pups drive to chase your cat is, he needs some sort of pattern interupt. You may need to get someone to come to your house to helpyou with this.

Some suggestions may be a squirt of water, this doesn't work well with gundogs in my experience, they love water, and your house gets very wet.

If he were my dog , I would have him on a lead as he goes to chase grab him by the collar and make him aware I don't find the behaviour acceptable, however I notice in your other post he sometimes bites you, so this may also not work.

I assume you have had him since a young pup, so for approx 4 months he has been chasing the cat, the longer it goes on, the stronger the behaviour, he really isn't aware at the moment it is not acceptable behaviour.

Good look, he looks a lovely lad.