Debi,
Your concern and efforts are admirable, and I also agree with the others that you have done what you could...

Jmo, but please be careful about falling into the roll of caretaker with him. It won't do either of you any good, in fact it has the potential to make things harder...
He has the financial means to seek counseling; he won't know if it will do any good unless he tries it. If not through the VA, then a private therapist with extensive PTSD experience.
Your letter was a good gesture, however you have no way of knowing if he actually gave it to the psychiatrist. He may have told you so in the belief it would make *you* feel better.

This is akin to trying to get someone to stop drinking--if they don't want it, there is no way it is going to happen.
You have provided your friend with several good suggestions on helping himself...if he chooses not to try any of them...there is nothing you can do.
Harsh words, but they are true.

The desire to help him is a good thing, but he has to take responsibility for himself. You said you were at your wit's end...it's a heavy burden when you want to help someone who does not want help.

It's essential that you take care of yourself first. Becoming entangled with his problems is going to drain your energy that is better spent elsewhere.

In no way am I saying to abandon him. But you may want to rethink this situation...what has happened so far has not helped him and it's causing you stress.
Perhaps there are other ways of helping that will protect you while shifting responsibility to him in seeking help.

In Peace

Catherine