Sounds like reverse reverb to me.
Reverse audio with reverb.
https://musicradiocreative-community.s3.dualstack.eu-west-2.amazonaws.com/original/2X/6/6555f686d8a3e96e86f4caabc1c4ec4daccd7fbc.mp3
He’s also slapping the 6th string with his thumbpick for the percussive sound. Check out Michael Hedges for more of this style of playing.