A series of discoveries and accidents led him to the "miraculous vitamin" or Miraclo. He found that concentrated doses of that "vitamin" gave a strength and vigor to test mice several times that of normal. Taking a dose, Rex found that his strength and vitality and speed increased to that of super-human levels. However, the human body could only process the Miraclo for an hour at a time, whereafter the subject was weakened and returned to human strength levels.
I was thinking that maybe the guy is somewhat 'vain', he keeps an illusion around himself ot look older than he really is, and before sundown, he hides from everyone.
Otherwise, as the sun goes down, the illusion ripples and peels away to reveal that he looks alot more youthful, and who'd trust a leader who looked younger than they are?