I would check to see if it is a rub maybe from his blanket, and if it isn't a wound or anything, it may be rain rot. My gelding and other gelding at my barn get it, especially during the summer. I'm not exactly sure what causes it, but clumps of hair fall out, especially during grooming. It's more like flakes, and the area of the skin is really dry. Apparently listerine (mouthwash) is supposed to help. I would try asking your trainer, and if you are really concerned, then have a vet take a look at it.