I was recently at a party where all the guests were friends of my husband. Other than my husband, I had met only the host of the party before (not even the hostess). However, since my stepdaughter shares an interest in the activity that brought this group together, many of the guests had met her.
There was one gentleman there who spent most of the evening exclaiming how much my "daughter" looked like her mother, meaning me. It is actually quite true that she looks like her mother; however, that mother is not me. Rather than embarrass him, I simply smiled, thanked him, and agreed that we were very proud of her (which we are) and didn't mention that I am her stepmother and she was in high school when I met her father; college when I married him.
Please don't think that we don't understand why you would want to know who your father is. We do get that, really. It's quite understandable. But our understanding does not affect the law. There is not, under the circumstances you have described, a legal *need* for you to know. There is not a law you can invoke that will force a court to compel this man to take a DNA test - there is not a law you can invoke that will get you into court at all. Your desire to know, understandable as it may be, is not a sufficient legal reason to force an invasion of his privacy. Which is what a forced DNA test legally comes to.