I use Skratch and Zephyr by Global Herbs on my mare who is allergic to tree pollen. It really helps. She has a nose net on all year round, and is about to be out in her full face fly mask now too.
best thing i have found is piriton. Works amazing allthough they do need a lot. last summer my friends 600kg horse was on 60 a day. cheapest place to buy them is the internet site chemists 4 u. £11 for 500.
The best thing to do is buy a pot of local honey and put a big splurge in every feed. this will help your horse to build up resistance and reduce hay fever symptoms. It has to be local honey as this will be collected from bees that fly in your area with the pollen your horse is allergic too. Give it ago. it works really well