Usually this is pain related - my first guess would be ulcers (if there's no obvious injury in this area). Is he disrespectful at other times? Does he only do it with stronger pressure or any touch at all? I would have a vet out if a typically happy horse started acting out with grooming.
Everything bothers him? That is something to think about. I'd hate to have a young horse that everything bothered. Do you mean things touching him? Or humans holding him? Being tied up? The last thing you want is to have a horse that everything bothers. You would want to work on those things before going on to anything else.