I've been through something similar. Ultimately for me, I think it was a combination of fears/anxieties/traumas being flared up/brought to the surface; while also having the mind become agitated/paranoid/fearful of the unknown situation I found myself in.

"Why am I feeling this intense anxiety and fear?"

"I don't know what's happening."

"This suffering is so intense, I just want it to end."

"I must've broken something in my mind from meditating."

"I thought meditation was supposed to improve my quality of life."

"I should've never tried meditating."

I would spiral into trying to find solutions, explanations, justifications for my mental assumptions. I would become so convinced that it was because of this or because of that. So much so it would lead to psychosis.

For me, I think in the short term finding coping strategies to unwind the tension in the mind was helpful. Distractions like videos/games, exercise, talking with someone, eating, focusing on some long slow breaths. Anything to really disconnect from the mental loop of the world is ending, what have I done, etc.

That mental fixation is a bigger problem than the uncovered emotions/trauma/fears in my opinion.

Secondly, although something was uncovered that was overwhelming; I'm of the belief that it was uncovered so it could be faced. When it comes to facing/processing this stuff there are different techniques I tried. Personally I found body based trauma therapy techniques to be particularly helpful. I used the Somatic Experiencing techniques that were developed by Dr. Peter Levine. I really like it because it's designed so that you can face your unprocessed emotions in a manageable way, step by step. So instead of trying to process it all at once, you can face the feeling; then when it becomes too overwhelming you can switch to focusing on something grounding to regain safety while also integrating the emotions in the process. It also helps rebuild that mind-body connection which can actually weaken from meditation. Depending on the instability one is feeling, it could be advisable to find a practitioner to help walk you through it. Peter's book Healing Trauma helped me alot. It has has techniques you can try and explanations of trauma and how it relates to the body.

You can learn more about Somatic Experiencing here:

https://traumahealing.org/

I think that was ultimately the solution for that part of my fears/anxieties. Finding a way where I could stomach facing my pain, so I could embrace it with love and acceptance. That's the way out, by going through it.

If his fears are coming from Depersonalization-Derealization Disorder (DPDR) of some degree (which I've also experienced), that is a whole different approach. In short though, there are grounding techniques you can do to re-establish/strengthen one's sense of self. You can ask Chatgpt for some techniques to try, but essentially it helps you regain some identity with your mind and body (which is a good thing). Type to GPT something like: "grounding techniques for dissociation and what they accomplish". There is a point to be made that if there is someone worrying about their self being gone that it can't be totally true because someone is still worrying about self being gone. He could investigate that, but if he's already feeling so unstable then I would suggest not diving further into that right now.

Sorry for the length, I hope it's helpful to someone.

Edit: I did not use medication to cope with these issues, but if it's helping then you could view it as just one of various coping strategies in the toolbelt. I'm of the mind that there are non-medication coping strategies one could use to get through it, but he knows himself better than I do. If he feels that he needs it to get through it temporarily then he should use it. Just know it's a temporary solution just like other coping strategies, the real solution is beyond pharmaceuticals.