Natural health: verruca and cat allergies

My nephew is allergic to cats, and whenever he comes to visit he seems to flare up in a rash. 

I try to clean all traces of cat hair before he gets here, but it doesn’t seem to make much difference. What else can I do to minimise his reaction, when he is here?

Hopefully, this is an allergy that he will either grow out of, or to which he will become gradually desensitised. His body is releasing too much histamine, due to a perceived danger or stressor.