Our DS has seasonal allergies as well, and if I recall they peaked for him when he was 3. Poor kid spent most the summer sacked out on the couch. If you decide to try the medicinal route, either Claritin or Zyrtec may work, but probably not both. Giving DS Zyrtec was like giving him water and had no effect. Claritin was slightly better and good when the allergies were "milder". On the other hand, Zyrtec has been a lifesaver for DD. With DS just plain old benedryl worked the best (we figured the allergies had him down so badly that he was a zombie anyway, if we gave him the benedryl he was the same zombie but able to breathe a little better
). Around age 4 it seemed to be the magic number for him and he suddenly outgrew the worst of them. I have no idea why to be honest, but now they're annoying whereas before they were debilitating. I'll definitely be trying some of the suggestions in this thread!
A couple of things to remember as well is that even though his allergies may be to the trees blooming, other irritants that normally wouldn't affect him can compound the problem when his body is fighting the allergen. Daily vaccuuming and sweeping, making sure to dust ceiling fans and surfaces weekly, washing in anti-allergen detergent, investing in anti allergen bedding can all make a huge difference as well. Always make sure he has a bath before bed - anything that may have been tracked in or clinging to his hair can make it worse at night. Also, check the windows in the rooms he frequents for if the weather stripping is old it can be full of mold (we got a nasty shock when he was around 2 1/2 and we went to open the window right over where he slept - it was horrible and we'd had no idea!).
Good luck - allergies can be really awful!