Is that on his side? The only thing I can think of is a crate injury? Kaiju had a sore/injury on his leg that looked exactly like this when he was younger. He's not a sore chewer, so I just assumed it was a crate injury and put neosporin on it.
Watch him closely and see if he chews on it at all. If he chews on it, he may have caused it. If he ignores it like Kaiju did, it's probably something from the crate.
Those are the only thing I can think of.
Of course, if it gets worse, vet trip is in order.