I know that I am a nurse, but my whole family has had horses my whole life, I grew up rodeoing, and worked for a veterinarian for 4 years in my younger days. Anytime you see your horse doing the things you are describing, this usually means that they are in pain. The coughing and watery eyes could be from the grain or hay dust, but the grunting and sweating is a sign of pain somewhere, and I would get ahold of your vet asap, and have him seen. There is no need to let him stand in pain. That was really observant to notice that he is doing this, he has no other way to communicate to you other than what he is doing, with the hope that you will notice and get him help.
Good luck to you and your horse, I hope you get him to the vet soon.