9.9 times out of 10, teeth grinding is a symptom of tension in the body, perhaps crookedness, pain/discomfort, or simply that the horse does not understand what you are asking.
I'd rule out a pain response first, make sure your saddle fits perfectly, bridles or bit isn't pinching, that teeth have been attended to within the last 12 months.
From there, it's a matter of good, tactful riding. Most teeth griding horses aren't off the leg. Get them more respectful of the driving aids, and the grinding stops.
Work with your coach if you are unable to find where the tenion is stemming from yourself. If you coach is worth what you're paying them, they'll be able to tell you where the tension is, why its there and how you can fix it.
Please don't just strap his mouth shut with a flash, thats a bandaid, the original problem will still exist and most likely will start to show up elsewhere.