I'm not a vet but I'm not sure that he has mouth rot. My little man Luie had something like this on his mouth once and it was not mouth rot. Check it out here in my blog. Jann
I used several creams at first, Zymox and then the silvadede (same thing you are using) then later my vet said to stop. It took a few months but if finally went away. I'm no vet but maybe you should use it for a while. Be careful if you are using it inside the mouth because it can turn the inside of the mouth and their teeth black