This is a long shot, but the National Archives, Washington D. C., in Record Group, for General Hospital No. 9, Richmond, Va., aka Receiving and Wayside Hospital, holds:

Registers of patients 1863-65 (ch. I, vols. 106-109, 124, 126, and 128-131); registers of deaths and personal effects 1862-65 (ch. VI, vols. 116-118)

These volumes are not at all easy to use. It is possible they may contain disposition of the dead but could indicate his ailment and period of hospitalization.

Worth a look see if you can get to D. C. or have a friend who could.