What does his body language say? Does he apear afraid? Angry? Defensive? Ears back alot? Tail clamped down? Is he flighty?
Have you changed anything in the recent weeks? And by anything I mean anything? His paddock...do you come at a different time to see him now? Does he have paddock mates? Has one of them been taken away/or one added to the group? Feeding change? Season change? Water?
There has to have been some recent change somewere to trigger this since he wasnt doing it before, and you have ruled out pain.
Purhaps as far as working with him in the mean time .....try and look at him as a brand new horse. Don't look at what he used to be like, just squarely focus on how he is acting, and try and figure out what purhaps caused it........ kinda as if he were a troubled rescue horse or somthing.
Just so you arnt comparing and expecting to much from him at this point, as it proberly wont help the situation.