I believe I have found his family (he was born Courjon but later recognised when his parents married in France).
Putting together a timeline: 1861-1865 (unsure exact year): Farge Achille Courjon marries Mary Domitilla O'Brien in India (search at http://indiafamily.bl.uk/ui/Home.aspxhttp://indiafamily.bl.uk/ui/Home.aspx ) (grandparents of Peter Fontaine?)
1871: daughter, Mary Domitilla Isoline born in Calcutta (mother of Peter Fontaine?) https://familysearch.org/pal:/MM9.1.1/FG3J-F68
1875: Mary Domitilla Courjon nee O'Brien remarries to Peter Alfred Bonnaud Feb 2nd at Calcutta by His Grace the Mosr Rev Dr W Steins SJ Alfred Peter second son of the late Peter Bonnaud to Mary Domitilla widow of FA Courjon (from fibis) https://familysearch.org/pal:/MM9.1.1/FGN5-MD6
1883: Mary Domitilla Bonnaud passes away in India. https://familysearch.org/pal:/MM9.1.1/FGGF-XNR
1893: Pierre Farge A Courjon born in London
15 Jan 1900, 10e, Paris (original at http://canadp-archivesenligne.paris.fr/http://canadp-archivesenligne.paris.fr/) Louis Raymond Roger Fontaine, born Pas-de-Calais (Saint Etienne?) 9 August 1866, widower of Pauline LaCoste, son of the late Georges Raymond Fontaine and Julie Requier (living in Brimeux), to Marie Domitille Courjon, born Calcutta, 20 Jan 1871, resident 35 Rue Caumartin, Paris, daughter of Achille Courjon and Marie O'Brien, deceased.
At this time, Pierre Farge Achille Courjon, born 3 December 1893, London, was legitimised.
1921: Mary Domitilla Isoline Fontaine de Mazingham re-naturalises in the UK, lists two children, not Pierre (http://www.nationalarchives.gov.uk/catalogue/Default.asp?j=1 )
1922: Isoline M. Fontaine de Mazinghem dies in London (freebmd.rootsweb.com)