Do you want an honest opinion? I would be looking for a vet asap to take a look at this. It is definitely not something I have ever saw, but from personal experience if it is not something common, chances are there is not much "at home" remedies for it. expecially when it comes to swelling. It could be caused by so many things. It is better to be safe, expecially if his eating habits are changing as well.
One question in the meantime. What do you feed him? And is there anything in the tank that he could of tried to swallow that had any sharp edges?