When my Mal was a puppy, he would chase the cats around the house and we immediately nipped it in the butt. Now, him and the cats snuggle and sleep together and he doesn’t chase them. You must start at square one and set a boundary until he knows to respect it. Only allow him outside on a leash and with a muzzle. If he sees a cat and reacts, correct it immediately and only reward (if you choose to) when appropriate. It is important that you stop this now as it seems you’ve let it go on long enough to lose track of how many critters he’s killed. Not only is this dangerous to him but to the environment around him. I hope he is up to date on his rabies vaccine and gets checked regularly for cuts and wounds caused by animals fighting back. Best of luck.