I hate to sound negative, but you may have to accept after a certain amount of time that he's just going to always do it. I recently met a pony who cribs like no other (every other breath, practically), and she's old and has been doing it for years. They've tried everything, collars and more turn-out and more work. Still does it. Like AppyButt said, it's a soothing behavior. They often learn it when cooped up and then, even later when they're in a different/better barn, always use that as their "blankie."

My mare came to me, as a former racehorse, a dedicated wood chewer. If she's turned out 24/7, she stops, but if she's in even for an hour she starts again. When she had to be kept inside for days at a time once (at a clinic held in a no-turnout barn), she literally tore her stall apart from the inside out. Completely destroyed it. This was after I'd had her for a solid year with ample turn-out, and she reverted immediately.