I had a mare who was kept in a stall with neighbors, but no other horses in with her, and she was forever hurting her legs getting cuts, scrapes, punctures, ect., and there wasn't anything in the stall that I ever found that would cause the injuries. She was in a pipe corral, she just somehow managed to hit something just right that she'd cut herself up. She didn't like where she was and that was her way of showing it. I even switched her and my gelding, and she did the same things in his stall. Honestly i'd say if there isn't anything you can find, and he's not severely cutting himself, then just leave it be, and get lots of betadine, and clean him up when you bring him in. Also I'd recommend gal salve, to help keep the hair its original color instead of coming back in white, or not coming back in at all.