My guess (without seeing the fish) is that he's bloated. The problem could be lots of things, two more common possibilities are overfeeding/constipation or MB mycobacterium infection (some call this Fish TB). There's a sticky about it.
Here's the link. If you think it's food, then fast him for a day or two, then offer green pea mush (the inside not the seed coat). If you're sure it's constipation you could try an Epsom Salt bath. That's stressful though. I lost a whole tankful of danios to MB. If he has dropsy type symtoms, isn't eating, has curved spine I'd suspect this. If he's doing ok you might want to get a UV filter so he doesn't infect your other fish. If he's clearly dying, you might consider humanely euthanizing him. But definitely quarentine him if he acts "off". I'll have a good thought for you.
Ok I just reread the 'couple of fish died' part. I'd suspect MB and think about if you want to invest $75 in a UV filter. Not good news, sorry.