Missing Piece: How did he go from a lazy tween to the driven teenager?

I was wondering if the cynical "carving man", was a representation, metaphor, of the Devil.?

Can you fly this plane?
Surely u cant be serious
I am serious,and dont call me Shirley

It was there... the high school coach told him that he just had a tough day, and he should "be thankful" for it, to which Brandon looks confused. He then tells him "days like today can keep a man riled up for a long time if he responds right." That is the key - Brandon responds right and uses this as motivation. The coach tells him if he ever wants to play a down for him he will have to be the first there and the last to leave. Brandon takes this to heart, as we see later on when he has grown and he is waiting for the coach at the gym.

This is all reinforced in the next scene, as Brandon spends a night with his errant father. He sees his father unable to stay sober even for a single night caring for a young boy - the same man who threw away his own talent and dreams when he didn't "follow his road" and keep playing music. This is clearly more impetus for Brandon's change. Finally, we see Brandon, in the car, scared of his drunken father who is raving outside the window, reach over and grab a pink book - what we later see is his Bible, and hold it close. It is subtle, but it is there. Add all this up, especially since they make up a sequential sequence, and I think it is pretty safe to say they showed the seeds of Brandon's transition and motivation. Next time we see him he has clearly taken these lessens to heart and changed his behavior.

As for the OCD, I also thought they showed plenty of that. Not only did he have to have things in its place, but they later show him doing things like only walking on the sidewalk (not the grass), and doing the same thing all the time, even to the point of walking through a construction site rather than altering his normal route. He also made his friends get back in dirty uniforms after having taken a shower rather than break a tradition of taking pictures once the tradition had been started, etc.

It was all there.

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Because he set goals that were big enough to inspire him to make the effort needed to achieve them.

The first goal was to play football for Arkansas. He wanted it so badly be put in the work to learn the necessary techniques, and the work to improve his body, first adding the needed weight and then getting in football shape, and put up with the teasing and the abuse from his teammates and coaches.

The second goal was to become an All-American, so he could get drafted by the NFL. He had to step up not only his own skills and strength and athleticism, but find a way to inspire his teammates to work as hard as he did, even though he wasn't a natural leader (just as he wasn't a natural athlete when he started out.)