I've been told from good authority (a fantastic bristlenose breeder) that if they eat bad food, they can get a bad infection in their tummies, which causes them to get constipated, swell and die. I've had 3 go like that (not all at once, thankfully!) and it's really horrible. Give the tank a really thorough clean, making sure you vac the bottom, where all the bad food sits.
You may find you can't help this guy, but by keeping the bottom nice and clean, you should be able to prevent it from happening again. If he is still eating, try some peas or something else that may help move his bowels along.
Good luck