M.J.
Try Peroxyl. You'll find it near regular mouthwash, but it's medicated. It's a rinse for dental sores. My dentist recommended it after I had some sores/scratches after dental work. I discovered it worked amazing on canker sores, too. Since it may be difficult to swish it around to that spot, I'd just put some on a cotton ball or Q-tip and get the area really good.