Solved!! Boy meets an old man who gives him a vest or something that slows down time around him. [s]

[Whitney]

This is a YA/Children's Si-Fi novel where a teenage boy meets an old man who never leaves his house and the man gives him or helps him find this vest type thing --or maybe it was a disk--that, when he wears it, makes time slow down around him almost to a complete stop. Consequently he can move so fast that people don't even know he is there. I remember in one scene he accidentally brushes a guys arm while he is wearing the vest and because he is moving so fast, it makes it so the guys arm gets a huge and nasty bruise.

[Whitney]

I figured it out. It's called Eddie Fantastic.