He was a lunatic who lived in the caves and the cemetery near Kheresa. He used to be in chains but he had managed to untie himself and was wandering around when he found Jesus. Amos did not have any demonic possession as he believed nor was he healed by Jesus but he had a mental affliction and abruptly changed his behavior when listening to Jesus. The story that he later believed, that the demons that tormented him happened to dwell in a swine, helped to consolidate his recovery. All the apostles except Thomas believed in the story of the swine. 
See also: UB 151:6.