Sadly, what you're already doing may be all you can do. sad

Do they just outright refuse to not smoke when your son is around? I know quite a few smokers become defensive about their smoking and not everyone believes that asthma is a valid illness...that it could kill him.

If you've tried to explain to them how it works, why it's important and they're all unwilling to cease smoking for a short visit, then you're already doing all you can. If you have no-smoking rules at your house, you can try to get them to visit you.

(And I happen to have asthma that is triggered by smoke as well and know how hard it can be and how big of asses others can be when they decide to get their hackles raised about it, so I can sympathize if that's the case. sweatdrop )