While this information is interesting and entertaining, it's insufficient to help locate this man in the military records. There are any number of William Williamsons shown in the C.S.A. index. For instance, there's one in Co. "B", 21st Alabama, which served in Withers' command at Shiloh. Another appears in the Confederate Guards Response Battalion, a Louisiana command which served in Pond's Brigade, as I recall. It would help to know if this man has been identified on the census for any state in 1860, and exactly where.

It's also possible that he's a civilian. There was no stipulation that a volunteer aide-de-camp be enrolled. In many instances the man in question did not belong to a military unit. Perhaps this man was simply an acquaintance of Withers and accompanied him during the fighting.

It's not the best example to cite, but Albert Sidney Johnston was accompanied by the Governor of Tennessee during the battle. He was noted as a volunteer aide, and if captured would have been described as a civilian.