Hi Emily:
Do you have the exact date of his birth certificate? If you do, you can write to:
Ministere des Affaires Etrangeres
Service Central d'Etat Civil
11 Rue de la Maison Blanche
44941 NANTES
FRANCE
Ask for a "COPIE INTEGRALE" to obtain the actual act and not an extract of it.
I am pretty sure you'll be able to obtain the document. If you don't have an exact date, just show an approximate year - no more than 5 yrs range for exemple.
Good luck.
Lucie