If he looks like a pinecone then chances are good it's dropsy. There really isn't much you can do other than keep the water as clean as possible and hope he recovers.
The link I posted suggests a strong anti-biotic that might work.
"Unfortunately, dropsy is *usually* incurable and fatal; however, in rare cases, spontaneous recovery may occur. A strong antibiotic such as kanamycin sulfate can be tried, but because it is an internal infection, usually it does little good"