They're octave harmonics. Lightly touch the string with your pointer 12 frets above the note that you are fingering and pluck with your thumb.
you know when you do a natural harmonic on the 12th fret? You can do that on the 15th fret if you fret the 3rd fret. It means you need to use your plucking hand to do it. That's exactly what he's doing.
I've tried it with my thumb like that, but it's pretty hard to get a sound...I think I need to grow a nail. Should be a lot easier to get a clear note that way.