There's several YouTube vids of him doing this on a 6- string. Sounds like he's tuned to an open chord - G maybe? There are occasional close ups of his left hand on his appearance on Live With Jools Holland, that show him fingering a chord shape that would be Am7, if he happens to be in standard tuning.
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On a six string he plays it in open D tuning.
If it's a fretted six-string CBG, here's a pretty good lesson. Good luck!
https://www.youtube.com/watch?v=hAGamyrbfnA
Anyone else see this? Any suggestions?