The Alabama Department of Archives and History has all sorts of Sumter County records that might help in your search.
Search on Sumter here:http://www.archives.alabama.gov/localrecords/search.cfm
Their listings include Estate Records, Land Records, "Miscellaneous" and "Historical" Records that could provide death info (such as his wife selling land as a widow) or relatives' names (listed in probate, etc.)
The records aren't online, though, so you'll need to conduct research in person.
Newspapers would also be a good possibility...