Since you have stated that he has an easy going, nice personality, I think you will be capable of breaking this habit:)
We took in my SO’s parents 5 yr old un-neutered lab when they moved across the country, and before they got him, his ‘breeder’(very loosely termed) owner had actually taught him to chase, and KILL(!!!) cats. Total a-hole, but I digress…
We also have a very beloved older cat, who was not going to go anywhere, and it was a very real concern.
The dog was neutered within a week or so of coming to our home, and although he didnt display any of the typical un-neutered tendencies(roaming, marking, aggression, sex drive) he was still going to be snipped.
When we first introduced him to Sylvester, he immediately raised his hackles, deep growl,puffed out and had a laser beam stare on the cat, who was on the couch. I had him on a leash, and he had to be right beside me and was not allowed to get infront of me. And I also had to keep trying to block him or give him a shove with my knee into his shoulder, along with me saying ‘leave him’ or ‘NO’ in an authoritive tone. Biggest thing was to break that stare and get his attention on me.
He’s actually quite a sensitive dog who loves to please and gets quite upset when reprimanded and is a total treat hound, which worked very well in my favor. And once he started learning that he was going to get talked to sternly and put in the ‘bad dog’ corner, which also meant he wasn’t allowed to be with the family, he picked it up quite quickly. He did go after the cat a couple of times, and I was present so punishment was swift.
Once the the cat was comfortable having the dog near him, the dog was allowed to sniff his hind end(cat was on my lap on the couch and SO had the dog on a leash). He sniffed without that laser eye and looked up at my SO with his ears relaxed and his tail wagging! We hit a break through! He was then lavished with attention and treats. Lol, he was so happy to see we were pleased with his behavior.
He slowly stopped eyeing the cat, which of course was rewarded and he stopped it all together.
He went thru a period of about six months where he was terrified to even look in the cats direction lol, if he and the cat had to pass each other in the hallway, the dog would squish up against the wall as hard as he could and not even look at the cat in fear he would get a swat from the cat, or a stern voice from us. Sylester had some fun with this, as he would sit at the bottom of the stairs, while the dog was at the top and he wouldn’t let him come down! He’d have to bark so one of us could save him.
They now snuggle together, with the cat getting the biggest most comphy spot of course, or will share a bit of food.
The dog even went as far as to protect the kitty this summer. I was cleaning my garage and had the cat and dog in there with me, when two of the Nieghbor dogs came running in(uninvited) and went after the cat and cornered him, my dog went ballistic and chased the two dogs out, and quickly ran back inside to make sure the cat was ok! That is when I knew for sure he had been successfully rehabbed from his cat killing days.
Breaking that intense stare and rewarding immediately was key for us. I second the leash thing, I would have it attached to my waist and I’d drag him around the house and yard with it.
Sorry this was so long, but i went through this exact same thing and had to share my success story. Everybody is harmonious.
Good luck with your new pooch! And keep us updated:)