The Boy from Mars by Robert DeLaurentis

The year is 2086.  The Mars Station, a cold and colorless interior city of ten thousand on the Red Planet is ruled by a ruthless Governor bent on creating a future dedicated solely to scientific advancement.  The population includes several hundred children, all of whom have been genetically designed…except for one. Fifteen year old Thomas Knight was the last child born on Earth and sent to Mars as an infant to escape the floods that ravaged the planet.  He leads a dull existence on the claustrophobic Station, and lives for the nights when he sneaks out of the segregated Boys' Quarters to break into the Artifacts Museum, where he can feed his obsession with all things Earth-related.  Finding an old Webster's Dictionary, he collects mysterious words that form a portrait of the magical planet of his birth.

One night, Thomas encounters an older settler who informs him that he is the heart of a bold mission, conceived by the father he has never known...to save the planet he has never seen.