"Punished "Venom" Snake, also known as Big Boss, and formerly known as
Ahab, V, Big Boss's phantom, or simply Snake, was a mercenary
commander, formerly a combat medic for Militaires Sans Frontières
(MSF), who led Diamond Dogs and Outer Heaven as one of Big Boss' most
During his time as a medic, Venom Snake saved Big Boss's life during
the destruction of MSF in 1975 by shielding him from an oncoming
explosion, losing his left arm and the use of his right eye in the
process. Despite his sacrifice, Big Boss fell into a coma. Venom Snake
was then placed into an artificially induced coma, undergoing
hypnotherapy and plastic surgery so that he could act as a body double
for Big Boss. This was done in order to protect Big Boss from XOF
forces who had destroyed MSF nine years earlier."
Got this from this link
So basically, he was the doctor who helped in retrieving the explosives from Paz's insides during the helicopter scene, and was induced in to a coma as the boss already was in one (which was not induced), in order to protect him.
And also, about the piece of metal, Venom Snake got it from the helipcopter crash, on the end of the game there is a cut-scene that explains that.