The Animal Welfare Act which states:
an animal must have a proper diet, including fresh water
has somewhere suitable to live is kept with or away from other animals, depending on its needs
is allowed to express itself and behave normally
is protected from, and treated for, illness
Clearly they don't have somehwere suitable to live if they are not allowed indoors. You could try quoting the act to him - but he doesnt sound like the sort of person who would worry too much about that, sadly