From the BTB web site www.travelbelieze.org
"With the exception of cruise ship passengers, all visitors to Belize must present a valid passport before entering the country. Please note that driver's licenses and birth certificates are not approved travel documents and cannot be used to enter the country. Passports must be valid up until time of departure.
In cases where a visitor remains in the country for over 30 days, an extension of entry must be requested. Extensions are available for the cost of:
Up to 6 months for US $25
After 6 months US $50 per month"
You should be all set. Hope this helps.