Here's a video

I've studied a bit of theory a long time ago, and I know there's a name to call the types of chords he uses but I can't for the life of me figure them out. I'm not asking what the actual chords are, but the category of what these types of chords are.

Open chords with a capo? He uses sus4 chords a lot, maybe that's what you're after?
they look like diminished chords to me, I might be wrong, tho
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It's mainly open chords...he doesn't really stick to one particular 'type' of chords.

I see some minor 7ths, a dominant seventh or two, some regular major and minor chords and some simple chords with a descending bassline...

All in all, that song would be very easy to harmonize with in terms of the vocals for me. (This point is totally unrelated to your question, sorry.)
I think the sus4 chords is what I'm looking for. He throws it in a lot near the end of each phrase and I kinda like it.