The furnace person that I have do service HIGHLY recommended the HEPA filters. He doesn't sell them, so, it wasn't any incentive on his end. Further, my BIL who is a HVAC contractor swears by them. I just added one to my furnace. They both suggested I change them out much more frequently then usual...every 3 weeks, initially. And, to lose them totally in the summer, as it will restrict my air flow and freeze up my AC.

There are also stand alone air filtration systems out there, including hospital grade ones, that are a long term option.

I have 2 sister in laws and one niece that regualarly take claritin, other OTC drugs, etc. They are all 3 alive and well, without any issues from taking them.

Finally, I would suggest he get allergy testing as you might not be dealing with just cat allergies...it might be many other things, and those, too, should be addressed.

Final finally- some might remember my 4 year old was dx'd with allergies, and prescribed drugs. After a heinous 2 hour allergist appointment, complete with allergy test, Jonah was dx'd as having NO NONE ZIP allergies. None.