Also, a nice guy =/= a man who persistently texts and calls. He's hiding under the "Oh, but I'm so nice, I just want to talk to you" nonsense but he's not nice if he's making you feel uncomfortable. He doesn't deserve a Super Guy Award just because he paid for his appointment - that's a pretty basic achievement, after all!
Like xw5 said, point out to him how he's making you feel and if he really is a nice person (just a bit of a plonker), he'll stop harassing you immediately. It's possible that he actually does know that his behaviour is wrong, though, and in that case, you should take the appropriate action (stop seeing him etc).
Sorry - I just get annoyed when people who are NOT nice somehow get away with the Nice Guy label just because their behaviour hasn't fully escalated to criminal levels yet. Sure, we all make mistakes socially now and then so he could be nice but mistaken, but I'm not sure our genuinely lovely clients who find it perfectly easy to not harass sex workers even if they enjoyed their appointment very much would like to be lumped into the same category as the unpleasant/annoying guys.