You have to assume your son is at serious risk for a true suicide attempt. My sister was saying the same thing as a teenager, my mother just did this in October - both made very, very serious suicide attempts and are lucky to be alive today.
The one thing that always sticks in my head was the "successful suicide" attempt statistic among males diagnosed with bipolar - I've seen upwards of 30-37;. My son knows that I take any discussion like this 100% seriously and am vigilant about watching him. I'm sure this is just from my family experience; but I never take these comments lightly. He may not even understand that he may try something; but everything I see is a huge red flag for the planning portion. Be especially vigilant if he all of a sudden becomes calm and reasonable.
Long, I know, but below are my personal suggestions having been through everything but the smoking (difficult child was a tad bit younger than yours and despises the fact that I do smoke a little) - as you've heard before, nobody can diagnose here, but if your difficult child was like mine, he needed intensive medication management and it took time; but it kept difficult child going. My son was also days short of Residential Treatment Center (RTC) when he ended up in the psychiatric hospital program and we finally ended up on a medication combo that helped stabilize him:
First, call the psychiatric hospital ASAP and let them know what is going on - they are legally liable if something were to happen. Can you get your current psychiatrist involved?
If none of this works, here's what I would do:
A) You need a psychiatrist you can see ASAP. It doesn't matter what the diagnosis is - these docs are just more likely to be aggressive in treatment.
B) Take the list of docs as well as any other names that you have (go through the phone book if necessary) and start calling at 8:00 tomorrow morning. Leave numerous messages that your son is seriously depressed and at risk of suicide and that you need an appointment ASP. You will likely find a doctor that cares and get an appointment immediately. My son's current psychiatrist saw him when I took this approach and actually saw him after hours (and the name came off the BiPolar (BP) kids list) on the same day; even though his practice was completely full. Just be prepared to pay out-of-pocket and submit the claim to insurance. A bit pricey; but it saved my kid's life.
C) I agree with the others; don't worry about the smoking for the moment. You know if it's manipulation or not. What I learned is that appearing to be somewhat OK and rational does not mean that the kid really is. In other words, psychosis or other mental impairment is not always obvious. My son had no control when it looked like he did.
D) The last thing I did under this same situation was force interaction. It may not be what everyone else would suggest, but I did find that if I got difficult child outside for a few minutes, with a little exercise, he was a tiny bit better and it got us through another day. The other was constantly being in his face and trying to talk to him. It was always unpleasant (difficult child was not violent towards us at this time); and sometimes I wasn't quite so nice. Other times, we just did whatever he wanted even though he was treated as a prince just to keep going. What we found out afterwards is that he was listening even when he couldn't talk.
My son was almost completely catatonic at one point. In my difficult child's case, his reaction was to completely shut down instead of lashing out at others or himself; he still cannot relay emotions well. We spent months pulling difficult child off of roof tops, out of closets and from underneath his bed because he would try to hide. He would hide and literally sit with a black hood over his head and just rub his hands over and over and stare into space for hours (he was not trying to commit suicide and it was not because I mentioned being not so nice above!).
I do understand where you are coming from. It wasn't until I got very aggressive that we started to find some answers.
Most importantly, tell your son what you are trying to do to help. He doesn't need to know the details, just that you are seeking answers and that you love him. If you really think he is at risk for suicide, talk to him about it explicitly. You are not likely to give him new ideas.