Long form of the answer, training is a way to solve this dilemma. I don't know if there is a way for the cats to ever feel 100% around him, but there is a way for the dog to show he is 100% not a threat. If he is not acting in these ways around the cats, the cats will be able to relax a little to check him out.
I'm not talking about training where the command becomes more of a trick or something not to be relied on. This takes training to a standard so that commands are at least reliable four out of five times. Also a dog learning commands undergoes a behavioral change, as they begin to correlate what is and what is not desired in their domestic home. Dogs really don't want to make this harder or inharmonious, but they do need direction to know how to do that.
The basic obedience commands are trained to a standard are pretty much all you need. Be Aware though, that not training to a standard will not get you or the cats where you want to go.
NOTE: THIS ARTICLE DOES NOT ADDRESS WHAT TO DO IF YOUR DOG ACTUALLY WANTS TO KILL THE CAT. THAT IS, OBVIOUSLY, A SERIOUS SCENARIO. IT WOULD REQUIRE HEAVY MANAGEMENT WITH SERIOUS TRAINING OR ONE OR THE OTHER TO BE REHOMED. THERE WOULD BE A SLIM CHANCE THIS MIGHT HAVE A GOOD OUTCOME. BEST NOT TO TRY IT IF THE INTENTION IS THAT ONE ANIMAL WANTS TO ACTUALLY KILL THE OTHER. THIS DOES NOT SOUND LIKE THE ABOVE PROBLEM IN THE QUESTION POSED FOR THIS ARTICLE.
We don't train cats, but we can train most dogs to be respectful around your other pets. Call us, e-mail us, or fill out our interview form. We will be in touch to come discuss the best training program and plan for you and your dog (and sometimes your other pets as well LOL).