>>151206169 (OP)
Well, I would say it happen when his first body died, he got reborn in his clone and pretend to be his son, right about the time of Superman's 'death', since he canonically stopped being fat in the main universe. It's worth the mention that he was fat only briefly before that, because his first incarnation was never actually fat.
I would venture further that new versions of the character probably went with the look of modern billionaires from the start, and modern billionaires obviously have the money AND time to be in shape.