I've had anxiety attacks like these (and full-blown panic attacks, tension headaches etc) for years, some just a sense of dread and unease, some really terrifying and debilitating.
I find that positive relaxation, visualisation, warm baths, massage, deep breathing exercises, gentle stretching exercises.... I used to feel offended that people thought these things would help, as though they were belittling the scale of my problem - but together these little things really do add up to quite an arsenal against anxiety.
I also recommend making sure that his diet is as plain, predictable and healthy as possible, and that his sleep pattern is as regular as possible - I know insomnia is a swine, but going to bed at a regular time and concentrating on physically relaxing each part of the body in turn, while visualising something that makes him feel safe/relaxed - it sounds naff, and it feels pointless at first, but it started to make a difference to me eventually.
He has my sympathy, it's awful. And you have my sympathy too, it's so difficult wanting to help the person you love and not knowing how to.