The only Manning's on the list I have are "George Manning and John Manning", both at 8th and Howard.
One of the articles from 4/23 states "200 bodies are cremated- This work was done by the authorities- numbers of bodies being intered whose names will never be known.
Two hundred bodies were found in the Potrero District were cremated by authorities. Some of them had been killed by falling buildings, some were burned by the fire that raged and some were killed by bursting water mains. It was thought the cremation was necessary for sanitary measure".
Do you by any chance live in California? If so, you can request an Inter Library loan from the California State Library in Sacramento, for the microfilm of a San Francisco newspaper for around the day your grandfather was born. The vital records of births section may list the name of his father.