A:It's tricky to try and diagnose a problem without seeing it. My guess is that your horse is either bored or has some discomfort. If he does this while standing he's probably bored. Keeping him busy--his mind and body-- will help, but it's unlikely you'll resolve the problem completely.
If discomfort is causing the problem you may try addressing dental and chiropractic issues. These are two common problems that can reveal themselves in many different behavioral problems.
Of course, if he doesn't have shanks to lip at, he can't do it. Try a bit that doesn't have shanks if possible. You could consider using a bitless bridle like a side pull or a Dr. Cook. A hackamore with a shank may or may not solve the problem. Shanked hackamores can be very severe as well--so careful handling is needed when using them as they can cause damage.