@Sharon1157
"Long term stays" have payments / payouts in monthly installments, so nothing can be changed to that.
If it is acceptable to you he can book for 1 week and then extend the reservation every week ("change "), so then he is weekly charged. Please note Airbnb Guest Service Costs will be higher (as they are lower percentage for "long term stays")
Also monthly discount (if any offered by you) is not applied, allthough you can correct price manually in the "change" form. Also the "long term stay" cancellation policy is not applied this way.
Please note it is not common to pay weekly on long term stays, also if it is not arranged via Airbnb.
Best regards,
Emiel