Need to help my son

My son is 21 and experiencing anxiety that is effecting his life. He is a bright young man attending University in another part of the country, but his anxiety has prevented him from passing more than 1 course (out of 9) in the past year. Of course I am not upset about the credits, but worried about his health. I know his anxiety has been building and he can't sit through classes, he avoids going out with friends and has extreme problems with sleep and eating. He also can't handle crowds, flashy lights and long car rides.

He could take a semester off school to work, but he is growing anxious about commuting to work or being able to work in general. Whenever we try to talk about the future, he ends up in his room not feeling well.

He is withdrawing from almost everything right now and I am scared. I've already had to fly across the country a few months ago to help him through a dark time.

I am quickly learning more about anxiety and believe this all may be stemming from a traumatic period a couple years ago when he sustained a head injury (concussion) and was unable to walk straight or have any short term memory. Many of the things he is anxious about right now are common to situations where he experienced pain, worry or incident.

Knowing a bit about what might be going on is one thing, but supporting him with what he needs is another. At this very moment, (2 days before Christmas) he is home from school. I'd like to have him stay here and work on his issues, but he wants to go back to his school town and try to sort himself out there. At 21 he is an adult. But he is still by son and I would not say that he is standing on his own two feet very well at all. We have tried to encourage him to stay home and work through his issues with the support of our family but he would rather leave and go back. Should I pull rank or let him sort himself out?