It seems that blue bettas are more likely to get it or at least, all the other cases I have seen on here were blue. I think the theory has something to do with overbreeding of blue's. Then again, red's are also very common too...
Anyways, you said he has been like this for 2 months or so? All of the others we have seen on here have had a sudden onset of this mystery rot and all were dead within 24 hours. That's why I asked about salt, I was thinking maybe it burned him. or sometimes a heater will break and the fish gets a chemical burn.
I think I recall someone fighting it off with Kanaplex. However I have never seen it in stores, only online. maybe triple sulfa or furan 2?