No, I did not say they will charge him criminally.
I have said that I do not know what they will do. I can't know. There is not a military law play book that forces a commander down any path. They have the legal discretion to go in many directions and neither of us have enough information to know what they are doing.
Statistically speaking, they will not charge him criminally. I say that only because criminal charges are the least likely result. It is more likely that they will simply use the fact of his AWOL as a legal basis for an administrative separation, like we've been talking about.
The fact that they took a statement from him does not necessarily change that as a potential result, but they can technically seek criminal charges. I don't see that as a particularly good choice for command. I would advise them against it, but in the end it is the commander that makes the choice on whether or not to charge someone. It is higher level commanders, with the benefit of JAG advice, that decide on how far those charges go.
The best thing that you can do for your son right now is continued contact, working through the psychiatrist, chaplain, etc. to get information. However, the command doesn't have to inform him of their decision until they actually start going down the path.
When, and if, he is appointed an attorney by the military, you'll then know that it is time to find an attorney for him. You can certainly look for one now, but that attorney won't have the legal ability to do anything unless and until he is charged. He has no legal right to an attorney until he is actually charged, so you'd be hiring an attorney that can't actually gain access to him.