Hmmm..it's probably just his coat growing in funny. It should be fine, as long as the skin/fur isn't discolored or swollen. Someone might have even shaved it for whatever reason, or rubbing against something, or whatever.

Have you wormed him? I would do that soon, I really doubt that its bot eggs but you should worm him anyways. Make sure you get something that kills bot eggs, they're quite formidable in the fall (at least with my horse). You can pick some wormer up at the feed store for about $10.