First of all, check that out.

And then see if it's a matter of someone not putting it in right in the "coding" area. I don't know the ins and outs of "medical coding" but I know with one of my mother's hospitalizations, I took 3 rounds of trying to get the insurance company and the hospital to speak the same language when "coding" the bill for what was a covered condition/covered benefits all along. Yet there were two denials, all because someone made clerical errors.

Regarding who is financially responsible, if he's over 18 AND you haven't signed papers saying you would be responsible, it's all on him. Sounds like a harsh burden, but he's young, one option would be to wait the requisite number of years and then just have it discharged in bankruptcy.

doc's offices and hospitals, are better at the 'coding' than others.

approved treatments for that are all rehab facilities.. not a locked down in patient psych unit.. apparently.

The paperwork we got WITH the plan says
"Unlimited days, no co-pay" for addiction services AND psychiatric services. The phrase that's not in the paperwork they give US is

"to be determined by medical necessity"

THAT apparently is where the small print lies. THAT'S where the real "death panels" are.

where he/she says it was medically necessary.

Talk to the doctors, talk to the hospital's insurance/billing department, go to a supervisor if you can. If the hospital is larger and has a patient ombudsman or patient advocate, talk to them.

Explain to them that 1) it was your understanding at the time of admission that their services and facilities were in-network (if applicable to the insurance) or otherwise met the insurance plan's requirements and that you and your son relied on their professional judgement about the necessity of this admission and it was your understanding that this was a covered service/treatment and 2) their ability to get paid in a timely manner, for a relatively large amount, is going to be dependent upon insurance paying for it, because your son and/or you can't afford to self-pay for it, so they better go to bat for you and get this mess straightened out.

Bottom line, it sounds to me like the insurance company is trying to weasel out. This is actually exactly what happened with my mother's situation. The hospital coded the bill wrong, and then the insurance company denied because they said the admission and treatment weren't medically necessary based on the diagnosis. BUT, at that time, I didn't know that the coding was the issue, so I raised holy Hell with the doctor and the hospital and told them they damned well better get it fixed. And, because of the issue involved, cardiac diagnosis, admitted for fluid retention, very low blood oxygen, etc., it was brutally obvious to me that it was "medically necessary". So the doctor did actually write a statement of medical necessity, then they sent it in and it was denied because they said their review people didn't agree it was necessary under the diagnosis. That was the point at which somebody got smart and figured out it was messed up. So the hospital people fixed what they botched and resubmitted and it was approved right away and paid. It had me freaked out, because it was a $15,000 bill for 3 days of inpatient care in the cardiac unit.

You are going to have to summon your courage and strength to fight with them, and you are going to have to MAKE the idiot doctors and facility fight on your behalf. It's hard, I've been there. But don't just take this lying down.