I agree with the above posts.
It could also be caused by him rubbing it on something, like a crate, fence, kennel, etc..
The small circle part of it, when zoomed in on almost looks like a burn, a small round burn like a cigarette or something??
If it isn't getting any better after a few days &/or seems to be bother the dog in any way then I suggest you take a trip to your vets.
Good luck.