Terrence Malson

Terrence Malson was a billionaire who transferred his mind into a small child named Jeff to prolong his life.

Biography
At the age of 16, Malson would be able to control Jeff's body however the Doctor kept erasing Jeff's memory to prevent Malson from taking control. Jeff eventually shot himself in the head to prevent Malson from taking him over, Malson disappeared soon afterwards. (ST: Running Out of Time)