As you can see, @Dionna0, this issue evokes very different opinions from hosts. If you read other discussion threads on this topic you will see a similar divide. Both @Huaai0 and @Rachael26 are very knowledgeable and experienced hosts and they have a different take on how to approach this problem. The salient issues I gathered from your post are that, 1) this is a long term guest so you will not be able to just wait until he leaves to be able to be comfortable in your own home and 2) this is a behavioral, not medical or cultural issue, as he had enough forethought to be able to shower before attending a meeting. Finally, while a gentle discussion with him seems to be the kinder action, I suggest a message via Air BNB will not only provide a trail of your concern, but also might allow him some privacy to consider your concern.
It is my thought that if this guest wants to use the Air BNB shared home option, he would need to adjust his behavior. I would assume (and hope) your review of him would be accurate and honest about his offensive body odor and personal hygiene. BTW, did he have a profile with reviews? What if you gave him a thumbs down review based on this and you never gave him an opportunity to correct the situation? That would not seem fair to me. Hosts complain all the time that guests leave negative comments without ever informing the host and allowing the host to correct the situation.
I don't think it is possible to have a " house rule" for every behavior that might arise in a share home situation that becomes intolerable for you. And certainly long-term versus short-term requires a different set of hosting skills. Perhaps you will consider a two week maximum stay for the future.
I am a believer that I learn my most important life lessons from difficult situations. Hosting, especially a shared home listing, has provided me with a number of opportunities to grow and confront uncomfortable situations. Best of luck dealing with this and keep us posted how it all plays out.