it’s called derealization or depersonalization depending on what he’s feeling. derealization is the feeling that everything around you in the external world isn’t real, while depersonalization is the detachment from yourself, feeling that you aren’t real. they’re both symptoms of a bunch of different things, most commonly anxiety disorders and depression.
tell him to google it sometime, read up about it. that’ll at least make him feel less alone. then he can look into meditation and yoga, that’ll help him stay grounded. there’s also cool sensation exercises he can do like spinning in circles, mirror staring, hyperventilation, anything that emulates the feeling of derealization in a controlled environment so he can practice feeling normal and calm while it’s happening. over time the feeling won’t scare him as much, it’ll bother him less and less, and then won’t bother him at all. this is called exposure therapy.
read through that PDF, the suggested exercises are at the bottom. i’d suggest mirror staring, hyperventilation, and spinning and stay away from anything with strobe lights.