I have a client in my business who is a christian clergyman. He goes by Pastor Tim. I just call him Tim. I don't think he likes it. I feel uncomfortable giving recognition to his title, but I want to know if it is actually a halachic problem or not. Thanks.

I found that Rav Soloveichik, in his letters, addresses even Catholic cardinals by their complimentary title, so apparently it's permitted. How much more so "pastor", which just means "shepherd" which is not necessarily a religious or complimentary title, would be allowed, and so agreed R. Shlomo Aviner.