This is Eric Stoltz as Marty McFly before Michael J. Fox played Marty's part. The creators of back to the future actually claimed there was more footage of Eric Stolz. This is the only resurfaced through the 25th anniversary DVD. But there is more.
The video provided by kaden2005 has since been deleted, so I am linking to other videos showing Eric Stoltz in Back to the Future. Copyright belongs to Universal Pictures. (I am also looking for early scripts of Back to the Future.)