Cage(1989)

A GI in Vietnam saves his buddy's life, but in the process is shot in the head. The injury results in brain damage to the point where he basically has a child's brain in a (very large) man's body. When they get out of the army the two open up a bar together, but some local gangsters make things tough for them after they refuse to take part in brutal "cage" matches where fighters battle to the point of serious injury and/or death.