If you have french relatives you are trying to trace some bdm records are now available if you have world membership. I purchased a pay as you go for ten looks at records. I am trying to track down a birth certificate for my great grandfather.
Oh, dear - I've just had a look, along with the new (I presume) Ancestry.fr site. It doesn't seem very helpful to me - all that came up for my (French) family name was a very distant branch in the USA. I suspect this is only the registers for Paris tagged onto ancestry.com. Never mind... I thought it was too good to be true, given the complexity of tracing French BMD.
For your great-grandfather, you need to work out which 'departement' he was born in and find out if that particular record office (archives departementales) has an index of its birth certificates. If it's indexed by 'commune', you need to know the town (or district) before you can start looking. My relatives in france have done most of their family history by post - very few personal visits - but they needed to work out the right town and/or region first. HTH
Sorry if I sounded negative, I think you've got what you need, in fact - the place name for his birth should be traceable (Wikipedia will give stubs or more for most 'communes' in France, with details of the 'departement'). Ancestry.fr has a link on it to the postal and web addresses of all the 'archives departementales'. I see Ancestry.fr has a link to Marne BMD, so that's one set of regional data there. HTH.