Hi Bellringer, you've got a similar problem to myself. The baby my Great Grandfather claimed to be died at 30 minutes old. (Shades of The Jackel?) I think I've tracked him down, but trying to find a birth certificate is proving hard. I've spent a small fortune on wrong certificates! I think that either my Great Grandfather was illegitimate or that he was ashamed of his parents (an Irish labourer and washerwoman if I've got the right people). Perhaps it's the same for your Grandfather.
I don't suppose your Grandfather had any siblings did he, or possibly cousins? If you could track down any of their descendents they may be able to help. How about the witnesses at the wedding? Do either of them look as though they could be relatives of your Grandfather?
Also, do you go on www.genesreunited.co.uk ? You could always post his name up on there and see if you get any joy.
Another thought - do you know the name of the Grandfather who brought him up? You may be able to track him through the census's.
Good luck!