Wraparound sunglasses. Wear a hat to keep pollen off the hair when outside.
I can't sit directly on grass, if either going to seed or recently cut - I come out in hives.
Dry bedding inside, which is sad I know but does help.
Shower at night to wash pollen off before bed (and wash hair every day), and I will wash my arms and face when I come inside as well, or even have a shower if it's particularly bad.
Sounds ridiculous, but do you live close enough to the coast to visit? When I was suffering especially badly as a teenager my mum would take me to the beach for the day, as the air there used to be much better and it would give me some relief.
I used to have to have time off school because of how bad mine was at times, so I really do sympathise. Can you push the doctors to do and allergy test to see exactly what he is reacting to? I looked at injection hayfever treatments where you inject yourself every day over time to get used to it. As an adult mine has improved enough that it's not worth it; hopefully your son's will improve as well.
I'd also look into stronger nasal steroids with the doctors.