Are the scales along his sides standing out any? That would be a tell tale sign of dropsy. It could be he's just constipated too though. I'd clean up his water and get any uneaten food out of there (it causes ammonia levels to rise), and then if you can get some peas, cook a few peas in some water in the microwave. Take a pea, mash it up a bit, and see if he will eat that. It's a natural source of fiber and if he's just constipated, it will help him pass his stool.

Other than that, I'd not feed him for a couple days until he's looking better, and avoid feeding him more than he'll eat afterwards. A varied diet (frozen brine shrimp and blood worms) along with pellets can help prevent it as well.

If it is dropsy, you can try getting something like Maracyn-2 to treat it, but the prognosis for it is not often very good. I hope you are able to help him and that it isn't dropsy.