I think demodex is a pretty good guess. It's possible in an older dog as well as a younger dog, though less likely in an older dog. If he's been stressed more than usual lately, it can cause a flare up of demodex. It does usually start near the eyes or on the legs.
I have seen some very strange presentations of demodex, so nothing is impossibe

There was one dog that had a chronic infection in his toe, and when we did biopsies it came back as demodex and staph. Then there were a couple shar peis that had ringworm-like lesions, and they came back as demodex and some other strange fungus I never heard of before. I don't remember the name right now.
Demodex is not contagious, so it does't matter if he's been in contact with other dogs or not.
And of course, it could just be a little spot that's irritated, and he's been rubbing it.