This is one of those situations where you're probably best letting nature do it's thing. It could be natural causes, it could also be from an insecticide that was used in the area, parasites, disease, and a whole host of other things.
If you were a vet with the right experience, you might be able to diagnose and bring him back from the brink, but beyond that, I think chances are slim here.
If you're going to hold onto him, I would rinse him thoroughly in room temperature RO water. You could also try soaking him in Pedialyte (search the forums, as I don't know the best practices/formations to use for this).
If he isn't alert within 24 hours, then euthanasia, or returning him to a nearby natural area might be best.