Masaru, like his friends, was likely an orphan by the time he was acquired by the government of old Tokyo, along with a group of other children. At the time, this particular government was spearheading a secret project, supervised by Colonel Shikishima, to develop psychic powers, possibly for militaristic and political purposes. This was where Masaru met and befriended Kiyoko, Takashi, and Akira.

The project ended when Akira grew too powerful to control and ascended to a higher plane of existence, leaving behind a psychic explosion that obliterated the old Tokyo in his wake. The survivors of this tragedy established Neo-Tokyo, while Kiyoko, Masaru, and Takashi, now referred to as the "Espers", were kept in prolonged isolation to prevent information about the clandestine project from being revealed to the public masses. The trio of children were constantly fed drugs to keep them from growing powerful as Akira, leading to premature aging while still young and developing physical handicaps; in Masaru's case, he developed polio and had to use a special wheelchair with a protective glass that accomodates his inability to walk (it is also used by Kiyoko as a transport in the anime)

(Source: Akira Wiki)