Glad you have found the right birth cert and confirmed that George's mother was Mary Ann.
Pity she was unmarried and in the workhouse, and as yet you have no idea of her age. I'm still drawn to the 17year old, so if you have Ancestry it still might be worth looking her and her family up on the 1901. Or come to that any other unmarried Mary Ann Shepherd or Sheppards in the immediate area of Brighton. (There was also a 26 year old Green Grocer's assistant I recall.) If the Mary Ann from Fulking (thats the correct name) is one you wish to consider, I believe that has been indexed as Folking.
Where to now?
You have an address in George Street which you have checked out on the 1901. I suggest you look a little closer at who was living there then just in case the wife was a married cousin or sister. So use FreeBMD to find the marriage of whoever was at that address. Their eldest child might give you an idea of the years you should be looking at for a marriage.
Hove (which has been called West Brighton in the past) also has a George Street, so it would be remiss of you not to do the same for whoever happens to be living there as well.
Let us know how you get on before we suggest further options.
The "My Brighton" website has several pictures and comments about the workhouse, and there is even a silent movie made circa 1902 which contains a few scenes where the workhouse is used as a backdrop. It is called, "The Soldier's Return" and is held by Sussex Film Archives. A frame from it can be seen on:
http://www.terramedia.co.uk/brighton/films_made_in_brighton_silent.htmRoy G