Whats wrong with my horse?

I just bought a 7 year old registered Quarter Horse gelding. He's really healthy and fit. Medically, there's nothing wrong with him; but he has a habit (don't worry, I knew about this when I bought him): he likes to stick his tongue out the side of his mouth. He does it all the time; wither he's relaxing in his stall or loping around the field. Any ideas about why he's doing this?