Loaned son-in-law $6000 to buy a truck in Alabama.

If you can't travel to Alabama, you will have to sue him in Florida (absent a default, you're going to have to go to court to testify to prove your case), but it is not clear that Florida would have any basis to exercise jurisdiction over him, that depends on whether and to what extent he has any contacts with Florida, e.g. does he own any property there, visit often, do any work there, etc.  Small claims courts are normally courts of limited jurisdiction in which you can't sue an out of state resident so, unless the Florida small claims courts are different (call the court to ask), you would have to sue him in a court of general jurisdiction.  One other possibility is if you have somebody in Alabama who would want to buy your claim, but you would still probably have to go to court to testify.