Ellen --
May 4th was the date that General Richard Taylor surrendered all troops in his department. The soldiers didn't know about the surrender until notified several days later. Major Dean's service file will include a statement showing exactly when and where he was paroled. The place may have been Byhalia MS -- the date was probably a week or two later.
If you don't have this officer's service file, you can always order it through the message board service you've seen advertised. That's also true for any pension file that may exist in his name.