I believe that Holcomb has his guitar tuned in what I call a "Open G Banjo tuning"

From the 6th string on down, here it is as follows.

String 6: G
String 5: G
String 4: D
String 3: G
String 2: B
String 1: D

On most of the songs that Holcomb did on guitar, this is the most common tuning.

If this helps, let me know.

Larry