I think that in an alternate timeline Doc Brown turned into a Batman kind of character and Marty was his Robin.
But you know, that is a wierd point I had never thought of. If Marty was a nerdy, science club guy it would NOT make sense that he would be hanging around Doc Brown, local genius scientist. Marty was actually portrayed a normal if rebellious kid, obviously well known as being a slacker in school, implying poor grades, absenteeism, and lack of respect for athority. In fact, with the introduction of the whole 'don't call me chicken' in the sequels, it was probably true that he was in a lot of fights and other problems.
Maybe Marty mows Doc's grass and Doc used the whole 'come inside, sweaty young boy and have some lemonade routine' so see a young Michael J Fox shirtless....
There's a hook for some scalding BTTF slash fic!
I feel so dirty now...