Evening!
Unfortunately there is no easy way to check for a later marriage/death - records will not have been put online and French law restricts access to more recent records.
His death certificate may refer to a spouse and often the informant/witnesses on records are relatives. It is also possible that a birth certificate may have what are called "marginal notes". These are cross-references to later records and may for example list a marriage or death. The absence of a marginal note does not mean there was no marriage.
The best place to look for records is the departmental archives. For St-Cast, this is Côtes-d'Armor.
http://archives.cotesdarmor.fr/index.php (choose "Archives En Ligne")
- it is now working for me. Unfortunately they have not yet put 1909 births online for St. Cast.
They do have censuses ("Listes nominatives") but these are not name indexed. I believe you will find Charles in St. Cast in 1911, in page 38. The family at that time appears to be:
Marie Conan, 25? (May be 35?)
children
Marthe, 13
Constance, 8
Charles, 2
Under "Registres paroissiaux et d'état civil" you can find some records relating to this family if not to Charles b. 1909 directly.
Constance Conan was b. 3 March 1903, the legitimate daughter of Charles Conan aged 36 and Marie Plessix aged 28. Marginal notes show she married first in Paris, 15e, I think 1922?, husband Felix Charbonnier, a second marriage is mentioned in 1973, she passed away in 1976.
Marthe Conan was b. 25 Mar 1898 to the same parents, she married in 1920 in Suresnes to an Alphonse Robert, and again in 1929 in Suresnes (difficult to read name, B? Boitrer?), and died in 1981 in Saint-Ouen-l'Aumône.
This family are together with another daughter, Marie, in 1906 (page 19, "Listes nominatives" for St Cast) - so either Charles Conan Sr was away in 1911 or he died sometime between fathering Charles Jr and the 1911 census.
The fact that older sister Marthe was married in Suresnes is a good sign as far as the Charles b. 1909 St Cast being your Charles who had some connection to Suresnes later in life.