Actually, Tim would have been the perfect middle linebacker since he stated that he just liked to hurt people.
When they first introduced his character, this was my initial thiought. In fact, he reminded me of the guy who played Lattimer in the movie
The Program. Although I still think he would make a perfect linebacker,
I'm OK with Tim as fullback because there is a natural rivalry with the running back position, especially at the HS level. Tim probably got the ball a lot in lower levels, then when Smash came along as a faster, better RB, Tim got moved to FB. He could have been TE, but as FB his main job is to block for Smash and it would eat at him more, and create the resentment we see.
this actually makes a lot of sense. Tim could be resentful and bitter because he could believe that Smash took his spot. Maybe this was the angle the producers went for when they had Tim's character in mind.