He's stuffed himself with too many dried bloodworms. I forget how big his mouth is and he just sucks them in 
He's had this in the past. Last time it was tubifex 
I'm currently nursing him in epsom salts and have given him fresh food and some frozen peas.
Please keep your fingers crossed for him because we've had him for about 8 years and we call him our lucky fish. His name is Jim and he's lovely.
Please feel free to say a < prayer for him if you like so that he might pull through. I'd be gutted if we lost him because he's so healthy and well.