If he's pastured with or near other horses they are likely the ones causing the trouble. Horses tend to bite each other a lot, even just playful nips. There's stuff that you can get for when your horses legs have to be wrapped (I can't remember the name right now) but you have to be very careful not to get it in your mouth or eyes.
I've also heard that hot sauce can work with some horses. I tried it once but they just stood there licking it off.
"You know, for as long as I can remember, I've had memories." ~Colin Mochrie