not just salt, you need Epsom salt. soak his food in an Epsom salt solution or if you feel particularly proactive make up a solution in an eye dropper and force feed him the solution. I've done that with fish before. Epsom salts won't hurt the fish and I wouldn't think it will hurt a snail.
I've never had a poly that would eat them, but skinned green peas work as well for bloat.
I've never had a poly that would eat them, but skinned green peas work as well for bloat.