Read David Swenson’s reply:
Anxiety attacks can arise from a variety of situations. I do not know of individual asanas that would bring them about. Each person suffering from an anxiety attack may find that a unique circumstance triggers it.
The first thing to ask is whether your student has experienced this before. If so, what was the circumstance? Was the room overly crowded or hot that day? Try to help the student narrow down exactly what brought it about, so you both can try to avoid it in future classes.
If anxiety attacks are a recurring experience for this student, then I recommend that he seek advice from a doctor or professional. Meanwhile, it’s great that he’s finding relief from his back pain through yoga. With a little research and inquiry, I am sure you can work together to find a way for him to maintain a regular yoga practice.