The marriage certificate could be the way to go, but as he took the adoptive family name and was possibly named after his step father as well...the chances of him giving his birth parents name is remote....but worth a try.... CHEERS!!
He may not even have known his birth parents' names. The chances are, if he was handed over as a young baby/child, that he was illegitimate. I would expect him to name his adoptive father as his father on his marriage certificate, especially if he used that name.