He sounds much like Riley did. Riley was our GSD/Border collie mix. Riley always watched the cats, even if it was just out of the corner of his eye. He KNEW they were off limits pretty early on. There were times he would still try to chase them and times you could hear a growl out of him but otherwise, he never tried to hurt them. Riley was wary of the cats.
Just keep an eye on him. Keep enforcing the leave it and make sure the cats have a place to go away from him that he's unable to follow if they want to get away. The fact that the cats swipe and hiss and he backs away tells me a lot. It really does sound like it's just curiosity and he's trying to figure out the strange dogs OR he's been around cats before and they were friends so he could be unsure why your cats don't want to be friends with him. I know my current pup just hit 9 months old and he still cant figure out why our cats don't like him. He tries so hard to make friends sharing toys and attempting to convince them to play. They just wont have it.
Bear in mind it can take anywhere from a week to 6-8 weeks for a dog to show it's true personality in full in a new home. But really, from your description, it sounds like he's just curious about the cats and is respectful of your wishes when you tell him to leave it.