Tbh, the most I personally believe is he is the living will of Rocks (like Luffy is to Roger).
I actually really like the idea that Dragon is Rocks' son and Garp took him in during God Valley. Makes a lot of sense when you realize that Dragon does not behave like Garp or Luffy, and that Garp never calls Dragon his son.
I.e. Garp had a daughter and Dragon simply took on the Monkey name.