Good point on the possible infection. A good indicator is the poop. If its brown, he's ok. But if its all white and clear odds are he may have problems. This isn't a for sure thing- just an indicator.
If you think he is eating too much, make sure you aren't feeding him. Feed other fish in the tank less so less food makes it to the bottom, and don't give him any bottom feeder tabs or anything.
Internal infections can be cured by boosting your water temperature or medications so you aren't screwed even if it is the case.
I doubt it though, odds are he's a piggy. Doubt he'd die from it, just put him on a diet.