Have you tried an open field shelter in his field?
Toys or other things for him to do in his stable?
A companion?
Otherwise a behaviourist or other trainer may be able to help you. Intelligent Horsemanship trainers are trained to look for the causes of behaviours, and ways to solve behavioural problems. (Other techniques are also available - eg., Parelli, Warwick Schiller, many others - and may suit you better. Have a look at a few websites and videos, and see what gels with you.)