I won't be as hard on you as others (although the fishes lives are in the keepers hands) as we all make mistakes.
That does look like a severe case of mouth fungus and needs to be treated ASAP. The fish may or may not be too far gone to help, but still try to help him.
I would consider salt treatments, Melafix, Pimafix as well as the Indian almond leaves, as milder treatments which would be sufficient for milder cases. But seeing as this case is severe, a stronger med is needed.
If you tried the milder treatments, they are probably not strong enough at this point and will have little to no effect. So you would need to resort to using a stronger med, but they are more harsh on the fish as well, and the fish may be too weak and die, but it's the only chance the fish has.
Not sure if you can, but maybe half dosages (wouldn't recommend if it's a antibiotic) of the stronger meds can work. There may be some natural potent herbal medicine out there (not sure for gram negative bacteria) that are much less harmful to the fish and still a strong treatment on the bacteria (not as strong as the harsher meds, but stronger than the milder ones)
When using meds, it's advised to provide additional aeration (surface agitation).
Whichever you go with, treatment needs to be done ASAP to save the fish, so don't delay.
Water changes will help keep things cleaner, but won't have the fish recover by itself with the disease progressed as far as it has.
Here is a website on mouth fungus (a form of Columnaris)
http://www.fishchannel.com/fish-health/aquarium-fish-mouth-fungus.aspx
Not an expert on this disease, but I hear kanamycin (active ingredient in med) is effective and usually safer (Don't do half dosages with anitbiotics or they won't be effective at all, do the full strength and course)
Erythromycin is mentioned sometimes, but it does not treat gram negative bacteria, so it won't be effective on columnaris.
With the amount of damage already done, even after treatment the wound will be quite large and still prone to reinfection until it heals more (the fish may forever be disfigured, but can fully recover). So make sure to keep the fish in pristine water conditions to give it the best chance of recovery and less chance of reinfection.
I am not sure if this disease is contagious. If it is, I would treat the whole tank just to make sure all traces are gone. And putting already sick/weak fish into a new environment such as a hospital tank overloads the fish with stress and chances are even slimmer of recovering, but sometimes is necessary if the disease is too risky (dangerous) of having spread to other fish.