When people are allergic to cats, it's actually the cat dander they are allergic to. That is the stuff the cats slough off their skin all the time. Constantly. If you've had the cat awhile, your carpet, furniture, bedding, etc., is already loaded with this stuff. Unless you are prepared to take extreme measures (like parents whose child has asthma - get rid of carpets and upholstered furniture) then he will keep having problems. He could try taking a non-drowsy antihistamine like Claritin (which is now sold over the counter) or Zyrtec or Clarinex, etc, which are prescription medicines. They work for 24 hours. If one doesn't work, have him try another. They don't affect everybody the same way.