To break this habit will take a long time. Invite a willing friend and her cat over to help you. place the cat in a crate and the dog on a harness and leash. Lets the dog come up to the cage and sniff it, but he shows any signs of hostility, or you can tell he is about to show hostility, say a loud, firm No! and and quickly lead him out of the room. Once he is another room where he can no longer see the cat, wait until he is perfectly calm. You can use treats and work on things like sit and down while you are trying to calm him. After he is calm for a minute or two, bring him back into the room. Be sure you and your friend are relaxed, as this energy will transmit and help the dog. Repeat this over and over as much as possible. Once he can calmly sniff the cage the cat is in, have your friend take the cat out, stand up and just hold it. Repeat the same thing, but this time only let him stay if he doesn't show hostility to the friend holding the cat or jumping up and barking and stuff. If he is fine with this, have your friend kneel down just enough for him to almost reach the cat. Once he can do this calmly, hold him by his harness and put the cat on the floor. only let him move from his position is he shows absolutely no aggressive behavior toward the cat. Once he can do this calmly, hold him on a tight leash and do the same. After this is mastered, let go of his leash, but stay close enough so that you can still step on it if he decides to chase the cat. After this, let him go a little farther from you, and work your way up to no leash. Every time he shows aggression toward the cat in any step, go back to the previous step and work your way back up. Be sure to CALMLY give him treats and praise during each session. Best of luck to you and your dog!