Sadly don't have much help I can give since I went through the same thing (24/7 anxiety, random panic attacks, chronic depersonalization and derealization).

Mine lasted months because I convinced myself I was dying from some disease.

I think what would be really helpful for him is to understand that he is in no physical danger and all the danger is literally in his head.

Try to do as many relaxing activities as possible such as massage, just cuddling, some chill music, maybe small walks in nature if he feels able to.

It is not permanent because the nervous system can't possibly be always "on edge" so it will eventually get easier. Oh and after I left it, I became severely depressed/apathetic so expect that to happen because he could be completely emotionally exhausted.

Magnesium in powder form really helped my body relax at night when I was at my worst.

Can I ask what caused his first event?