We have same problem. It seems that Airbnb knows he's been using your account (cookies, logins, whatever) and so refuses to let him be a co host, because he's you.
We haven't sorted it yet either. But he would probably have to remove the app from his phone, and remove all cache and cookies from all Airbnb pages in his web browser, so that Airbnb doesn't identify him as you.
No, not something we want to do either. A lot of work.