Have you tried to get his social security application? This might contain the mothers maiden name if he knew it. As I recall I had to write my mothers maiden name on mine way back in 1944. His # 546-01-5499 issue in California.
If you have not found him on the 1900 census with his family, it might be that he is the one in Lincoln in the Home of Friendless, with Frank. A good library in Lincoln might have information on that institution.