@Pamela247, you can establish a rental contract parallel to the airbnb agreement, only you cannot add other terms nor restrictions, that have not been disclosed before booking.
As he is already here, you could discuss, what his plans are, how long he is staying in the region etc. You should get an idea of the risk involved. The limits for long term stay becoming permanent lease vary. In my country, it’s 90 days, therefore we limited it at 45, with rare extensions possible, but always under the limit. In your place, it could be 30 days, so look it up online. I would stay under that. You should not extend the first stay over the limit and not add a consecutive contract.
if the guy is a guest to keep, I would send him a special offer with a day in between and book that day via a friend at 20 bucks. With review. You can always let the guy stay on, but legally, he can’t proove it. To send him away for the night would be preferable. He may wish to visit his fam8ly during a weekend anyway and if he gets a better rate or a refund for one or two nights, he might love the idea. I would prefer to lose a few dollars and not bear the risk of gaining a permanent family member.