I have been studying up on respiratory issues and Blue Green Algae came up a few times. I am modifying his environment and thought I would try the BGA.
After a week of being on the BGA, his nasal discharge has completely disappeared (it's almost freaky that it went away in a week!) but he'd definitely coughing more under saddle. Like a dry, hacking cough. The temperature dropped by 10+/- degrees in the last few days and there are several other owners at the barn (regardless of whether the horses lived in or out) said their horses are coughing more too.
(Also, he recently had an allergic reaction to the new roundbale in his field [we pulled him out of the field once we realized what had happened] and has hives all over his shoulders and neck. Could that have something to do with the coughing as well? It did appear around the same time as the crappy roundbale....)
Sorry if some of this sounds dumb, I am just thinking out loud (sort to speak).