He's mostly playing blues, which means the I, IV and V chords of whatever key/tuning he is playing in. I chord is the same as the key/tuning he's playing in, IV chord is built off of the fourth note of that scale and V chord is built off of the fifth note of that same scale. So in the key of C, the I chord is C, the IV chord is F and the V chord is G--in the key of A, the I chord is A, the IV chord is D and the V chord is E. All other I, IV and V chords are derived in the same way. Where he is playing in an open tuning, like Spanish tuning or open Bb, the left hand will be altered because the tuning has been changed.
You can use the table that Slack provided a link to to figure out the appropriate tuning/position for each song, and the pitch at which it was played. Get into the tuning/position for a song you want to play, listen to the song over and over and copy the sounds until you're matching what you're hearing. Use your ears, and hunt and peck if you have to to find the right chords. If you learn a song that way, you'll remember it forever.
All best,
Johnm
