Ok, I think I have this one figured out:
There is a BROYLES, LEWIS listed as a PVT from Gooch's Mill, Cooper County and engaged at Boonville. So it appears that either the letter's author, the transcriber, or some contemporary record clerk made a small error. Lou Boyle = Lewis Broyles, an enlisted man familiar to the letter writer, probably not an officer.
I'm embarrassed that I missed this one before, even though I had him on my list...I got lost looking for a match with Bowls, Bowles, Boyce, Boyle, and Boyles.