As promised, an update.
I scrubbed away at his cheek with betadine + hot water, and got the scabs off pretty easily. To me, it looks like a cluster of bug bites, like something got stuck under his fly mask and nailed him a couple of times, then he rubbed it hard while itching. Barn manager thinks scrapes. We are both agreed that it looks ugly but not serious. He got Alushield on it and the BM will keep an eye on him.