What a GREAT suggestion! I've been so obsessed with the frustration of having the SSA turn me down that I haven't been able to think straight.
He was 15 and it was 1943. His parents were immigrants in NYC, so hopefully they were listed on a directory. He was in the Military, so we are trying military records next, but he probably listed his foster family as his contacts. Too bad he wasn't a little bit older, there would be a draft registration card.
Thank you for your advice. I'm going to try a City Directory and welcome any further suggestions!