1) Gather documents. Anything that the property management company ave you that states that the lease is ending March 31st will help.
2) I would start by talking to the management company to get their assistance and gather any other documents you can from them that will support their case.
3) Talk to the landlord. It is very clear that the lease was intended to be extended only until March 31st. Be polite, but make sure that the landlord knows that you will cause endless hassle for them if they do not agree.
4) Take him to court - http://www.nolo.com/legal-encyclopedia/security-deposit-lawsuit-florida-small-claims-court.html