He may like his drinking water slightly warmed - my goats drink loads more when I give them warm water. I also live in a soft water area, so no issues ever here (so far), but if you water is hard you could either see if your rain water is clean enough or get him bottled water (a bit over the top I know).
Other traditional remedies are indeed adding some salt, or some raw apple cider vinegar (which you can get for horses), but mine always balked at the cider vinegar...
Also ammonium chloride is the one thing that is added to tup feed to make it ok for male sheep (incl wethers), but as he is not on any concentrates that may not be applicable.
Difficult to predict which of the above strategies may or may not work (or combination thereof), but given that it is an incredible painful condition I would seriously think about his longterm prospects if he was mine.