Trying to fit story, name, dates and location with primary records, there is a James (Jas) L Mason Pvt Co D, 15th Arkansas Infantry, Captured Fort Donelson, Shipped to Fort Butler Springfield IL, then to 4th St Hospital St. Louis where he died 9 Mar 1862. He is listed as having been buried in Wesleyan Cemetery, St Louis but was re-interred at Jefferson Barracks Section 66, Grave 9907.

Findagrave entry: https://www.findagrave.com/memorial/21744664/james-l.-mason

Interestingly there is also marker at Camp Butler National Cemetery, Springfield IL for a Jas Mason Pvt Co D 15th Johnson's Ark Infantry with death date of 26 Mar 1862. https://www.findagrave.com/memorial/2557209/james-mason

Fold3 indicates these were 2 separate individuals with different enlistment dates. The first is listed as about 25 y/o. No age is on the second.

There is a James Mason in 6th Ky Cav. died of wounds in 1864.

So I've kind of exhausted my dive into CSA records. If he was working as a local civilian guide there may very well be no military record for him.

FWIW, JJR