Sounds like his skin is leaching serum or some other type of fluid probably due to knats or flies or some other pesky pest - check closely for lice, too! Anyway, if that's what it is, then he's being bothered by some sort of insect. If it's gnats, then you won't see them during the day - they hide out in shady areas daytime and come out to feast (blood-suckers) in early am or late eve and all during the night.
Try a good wash down with some dawn soap - get all the stuff out (so it won't attract and more bugs - IF IT IS bugs/just a guess) and drench him good with fly spray in those areas. Keep those areas covered with repellant and see if it pops up again. If not, then you know it was pesky bugs.
Something to try, anyway....!!