Makes sense but what I meant was how did he go from being bald to having a full head of hair?

By casting someone with a full head of hair.

Ok

A vigorous fitness regime and hair implants

Maybe he was just a body double or an actor to trick Desmond, as they wanted to make him believe it would be kill at this moment.

Makes sense

He befriended David Goggins and smoked a pack of menthols twice a day.

Hmmm

First he looks like in his 40's in a game and second he looks like in his 60's in a movie

@Catpocalypse in the game he was 61 and in the movie he was 65