Ditto all of the above.
I have one horse with environmental and food allergies.
He does not eat oats/corn/soy.
I tried the drug route for the environmental issues for three years and while, the TriHist helped a little, it amped him up more than it helped.
He's been on allergy herbs, seasonally, for the last four years and does fine with them.
He never had these problems until we moved to the Tennessee Valley where allergies prevail; unless a good wind comes along and carries the stagnant air away.
Get his teeth checked first, just to be sure