Are you sure he's neutered? Are you sure he's male?!?!?! :)
Seriously...this is hard without seeing him. Assuming he's not just getting *really* fat...most intestinal disorders make cats thinner, and the ones that cause bloating usually leave no appetite.
Can you tell me if you can handle him at all (touch him)? If so, press gently about the bloated stomach region. If it's solid feeling/hard...well, just let me know what you feel. Try to describe how the fur feels, the skin, if it feels squishy (not in a healthy way), and how he reacts to pressing. Try in a few spots (pressing).
Is his behavior changed at all, and how long has h been like this? I can think of 20 possible things it could be, but need to narrow it down. At least half can be cancelled out if he's been like this more than two days, since those things lead to a fairly immediate death...
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