It may be a good idea to edit this post to take out the names, just to avoid privacy issues for the rellies who don't want to talk. You can still get the point across without stepping on any toes! And they may not be talking, but maybe one of them has a photo album or scrapbook with some sort of a hint in it.
I'm guessing from your profile that he was born somewhere in the US before 1940? If you can pinpoint where he was born, or where his parents were living, you can try to use the closest census to his birth to find families with the surname in question. Look for women of an age to be a possible mother, and trace their family lines to see if any of them had other children, and pay special attention if any of them called a child for either of the adopted parents, or gave their surname as a first name.
Search local papers for a birth announcement. If his parents were prominent in town, there may have been a write up in the society pages about a tea or shower thrown after his arrival. If his family shows up in the society pages, see if they ever threw a party for any occassion and invited anyone with the unknown name to establish connections to research.
This may be a touchy subject, but if either of his parents had sisters, the sister might have been the "girl in trouble" and they put the supposed father's name in to try and shame him into marrying her or in hopes that he'd provide support.
Don't forget to check the registration date on his birth certificate, too. If he was registered a significant amount of time after his birth, there may be a paper trail. He could have been registered by the hospital as a child of the birth mother. Look for births around the same time as him, with the other surname. Depending on where you're looking for records, and if the person whose parentage you're researching is living, you may be able to obtain hospital records for their birth.