In nice weather he is allowed into a larger pasture and I thought maybe he was finding something out there that he was eating and having a reaction to. He has now been off that pasture awhile and the sores remain. Otherwise he has had no changes in his diet. I thought maybe it was something in his grain, but he has eaten the same brand of grain his entire life. My vet has been out to look at him and she too is unsure of what they are.
I know they are not being caused by irritants in hay (fox tails), vesicular stomatitis, blister beetles, and they are not ulcers.
They are very small ( ~ 1/8" in diameter if that) and look almost like cold or canker sores.
I was thinking they might possibly be caused by some virus or parasite but I can't find anything that seems likely.
If anyone has any advice or experience with anything like this I would greatly appreciate it!