Archive papers from Gov Flecther's records
https://mdh.contentdm.oclc.org/digital/collection/msa/id/1343/rec/187

This letter dated Dec 25, 1865 refers to Col B.W. Lewis while Lewis is still alive.
https://mdh.contentdm.oclc.org/digital/collection/msa/id/1007/rec/138

So to narrow the time frame from primary contemporary documents he is Mr Lewis on Nov 2 1864 and Col Lewis on Dec 25, 1865. There is no indication he ever directly served in a military unit so I'm thinking this is an honorific title. His brother, a legitimate Major, never refers to Ben with the rank title.

Interesting person and times though, good story!
JJR