How does it look today? Any change?
If there's no change, I think I would just keep him in warm, clean water -- and keep a close eye on it for a day or so. Hopefully, it'll clear up.
If it looks like it's spreading though, OR if his behavior changes in any way, let us know right away. Based on what happens, or how it looks, someone can help you determine how to treat it.
I'm a big believer in trying just warm, clean water first. Along with good nutrition (and reducing stress), that will go a long way in allowing his immune system to handle it on his own. BUT if it looks like his system can't handle it on his own, then medications would be justified....