Start with 1tsp per gallon. You can work up to three per gallon with Epsom salt. I would still slowly acclimate him to the salt. He may be more irritated with being in a smaller container than with the salt. Epsom salt is very mild and can be used long term in certain chronic-bloat cases.
I think that fluid build up causes it to be clear, yes. I also believe it is a side effect of internal organ failure, although it may not always be the case. If he is bloated and still pooping, it is not from constipation. The salt acts not only as a laxative but it can also draw fluid from the body. Watch closely for pineconing.