New York adoption records are confidential and under court seal. An adoptee who was born or adopted in New York, the birth parents or biological siblings can register with the Adoption Information Registry to obtain certain kinds of information. The Registry helps adoptees obtain avaiable non-identifying information about their birth parents and enables the reunion of registered adoptees with their birth parents and biological siblings.
You might want to consider DNA testing to find his adopted surname. There is a good article "Finding Your Birth Family with DNA", Family Tree Magazine, 12/17. Successful Searching!