I have an anxiety disorder and get panic attacks. Sometimes they come out of nowhere, sometimes I can feel them coming on, but I can't really pin down my triggers. It is one of the scariest things that I have ever experienced.
What I have found to be helpful is mostly what dionysis and mysticcat said, but i was also grateful when my friends would remove (or as best they could) me from the situation i was in, had me do whatever was most comfortable (walk, sit, or lie down) and to help me breathe. An exercise i found helpful was to slowly breathe in for four counts, let it out for four counts and then hold my breath for four counts. then repeat (obviously lol) water is usually helpful for me. There are a few books that you can check out of the library. I did a quick search on google and came up with some good sites (especially from nimh- national institute of mental health)
i did a search on barnes and noble, and while i haven't read this personally, my step mom (who suffers from panic attacks too) has and found it helpful
don't panic
i saw some others, but since I haven't read them, i don't want to recommend anything.
I hope that helps somewhat!
I would also recommend talking to him about it, ask him what he would like you to do in those situations and what you could do to help and support him 110% to go and see someone.
I found cognitive behavioral therapy to be effective.