@Andrew B. You are exactly right. He'd have to give notice by the end of this month for an effective date of May 1st.
@Joe Prenger if you're ultimate goal is to raise rent AND keep the long standing tenant I'd suggest a gradual increase rather than a large jump. Have them actually sign a lease agreement (month to month or otherwise) with a smaller increase every 3 months or so. I had several tenants I did an increase on. I created an addendum in the lease specifying the dates and the amount of the increase.
Here's a helpful PDF for MO Landlord/Tenant Laws.
Here is an exert from the PDF:
"ORAL AGREEMENT An oral agreement obligates the landlord and tenant for only one month. A landlord can evict the tenant or raise rent with only one month’s notice. Likewise, the tenant can give notice to vacate on one month’s notice. (One month’s notice means a full calendar month, and must include a full rental period. For example: If your rent is due on the third day of the month, your rental period runs from the third of the month to the third of the following month.)"
https://www.ago.mo.gov/docs/default-source/publica...