... tuned in 5ths, like a tenor banjo. That's why some of his chords look so wacky. Tiny Grimes was a famous tenor guitar player from the bebop era. He sounded a lot like Charlie Christian, but with only 4 strings. I think he tuned his like the first 4 strings of a guitar, though. Actually, I'll have to check on that.
Jimmy is using C, G, D, A tuning (same as a tenor banjo, also same as a cello). If you stop the video when he's calling out the chords, he's playing the C with two fingers ... 3rd fret, first string (a C note), and 2nd fret, second string (an E note), so with the two open strings you have the triad C, G, E, C.
Here's a color shot of the guitar: https://www.pinterest.com/pin/158681586842998340/
And more info than you want to know about tenor guitar: https://en.wikipedia.org/wiki/Tenor_guitar
... although I found it interesting as a guitar player. Lots of stuff I didn't know.
And yeah, Jimmy was the first inspiration to become a guitar player, although that wasn't realized until I was about 20. But Jimmy was the first one to give me the bug!
Wow! This is an incredible post giving the origin of Jimmie's guitar, photos of his collection, including a Martin tenor that he played before the MouseGeTar. I have not read it yet, but it is definitely bookmarked for later!
https://umgf.com/is-the-most-collectible-non-martin-guitar-a-4-stri-t148794.html
♫rog♫