I have a horse that has an itchy eye.  Flys bother it a lot and so have put his fly mask on and it helps some but then evidently it gets to itching and he rubs off the mask.  What can I put in his eye to stop the itching?

Terramycin.  Tractor Supply carries it.

My gelding gets same thing just in right eye every spring/summer. I feed the skin/Allergy supplement made by Santa Cruz animal health. Zero problems ever since. two little pellet/scoops a day. 

 give 1 generic Zyrtec per 100lbs once a day with there feed. 
My filly had a similar problem her eyes would also water.