From everything you have said, everything is pointing at allergies. An easy naturpathic method of desensitizing a peron's allergies is through the purchase of local honey, purpose grown around the worst local plants, allergy speaking. Heathfood stores carry them everywhere I have been. As for the allergy patch test, now you have clarified this as a seasonal allergy effect, I especially recommend having one done.
Maybe you have been through the process yourself with less than stellar results. I went through the patch test as a teen and reacted to nearly every catagory of alergen. My doctor suggested what would be years of shots and I suggested that I'll deal with my own problems. The flip side of this came when some mold/dust was causing life threatening reactions at home and work. Hoping the treatment of allergies had advanced in the twenty years since my last experience with an allergy specialist, I went in for an evaluation. It really has advanced a great deal. Molds are still a problem to desensitize but nearly everything seasonal has an effective and short term treatment.
This is a personal bias but I don't like antihistamines. More to the point, they don't like me. Benadryl and the like put me dead to sleep, "drowsy" is an understatement for me. Conversely the non-drowsy formulas hit me like a dose of meth. I have never had meth but a prescription that will remain nameless nearly agitated me to violence, over a donut! In years that I was really suffering, I took and became adjusted to a non-drowsy prescription antihistamine.
I guess, knowing that Zyrtec will help with his misery is worth the trade off. I am not up on pediatric treatment of allergies so a lot is based on second hand results. We all grow out of most allergies. The people who have really bad allergies, like your kid, don't seem to grow out of them and sooner or later have to see an allergist. Maybe now is a good time? Any and all nasal sprays will begin to loose effectiveness, it may take more and more antihistamine to treat allergies and then where are you? This is just a potential outcome, the die isn't cast, but as you say, this isn't necessarily a CF issue.
Hope things continue to improve,
LL