Could have been a simple bug bite. Him scratching it just makes it worse. I'd go to Petsmart and looking for an anti itch cream or spray. My parents dog licks herself out of control and ends up with rashes like those.
I'd get one of the hot spot sprays from Petco or whatever is close and keep him from licking it as much as possible. By Wednesday it could either be a lot worse or almost disappear depending on how much he keeps licking it.
If you can't find Vetericyn at a local pet supply, some drug stores also carry Puricyn Plus, made by the same company as Vetericyn. Puricyn is the same active ingredient, labeled for humans (found in the first aid aisle).
I use them interchangeably -- on humans or animals -- whichever is on hand. I can't tell any difference between them, other than the label.
From the mfr website:
"What is the difference between Puracyn Plus for humans and Vetericyn Plus for animals?
Puracyn Plus is indicated and FDA cleared for human use. Vetericyn Plus is indicated specifically for animal use only. Both products are based on hypochlorous technology and are manufactured under strict quality standards. The FDA does not provide directions for animal use devices. You can learn more about Puracyn at http://www.puracynplus.com/"