Maximum penalties cannot be defined in terms of years on juvenile offenses. Your son could be given anything from sanctions to porbation to a residential program until his 19th birthday which is when the juvenile court loses jurisdiction. Prior record is a large factor and the Department of Juvenile Justice will make a recommendation which will have great weight with the court. I would advise you to consult a local Criminal Defense attorney who is experienced with Juvenile matters so you can speak in greater detail. I would be more than happy to discuss the case with you.
If your son was charged with unarmed robbery under sec. 812.13(c), Florida Statutes, that is a second degree felony. If he were charged as an adult, he could receive a maximum of 15 years in prison. That is the absolute worst-case scenario.
If he is treated as a juvenile, there is a possibility that he will be deemed a "serious juvenile offender" under sec. 985.47, Florida Statutes. He is over age 13, and the crime he is accused of committing is one of the listed qualifying crimes for that designation. If he is determined to be a "serious juvenile offender" by the judge, he may be subject to about 9 months of intensive secure residential treatment, followed by conditional release. In any event, the duration of his commitment cannot exceed the maximum term an adult would receive for the same offense.
Unarmed robbery in Florida carries a maximum sentence of 15 years in state prison, regardless whether you are a juvenile or an adult. The first step will be a determination by the state whether to charge your son as an adult or as a juvenile. Many factors are taken into consideration before such a decision is made. This type of case is best handled by an experience criminal defense lawyer.