Okay, let me first say that I have worked with kids with many a diagnosis. From what I got from your letter it sounds like there may be more going on here. If I had to phathom a guss on the little information given, there may be some oppositional defiant disorder, or some borderline personality disorder (I am leaning towards this one with the exagerations you state). It is hard to do anything when you do not see him often, and I am guessing that the mother may have some psch. issues as well.
First things first, you and your husband keep a log of all his behaviors, this will help any psych. get a better picture of what is actually going on.
Second, PLEASE, PLEASE, PLEASE watch any medication that he might be on. Many psychotropic medications are not made for teenagers, and can often make sympots worse, or send him into a downward spiral. You have to remember the teenage mind is not fully devoloped yet. The pre-frontal cortex (which in addition to being responsible for working memory, is also responsible for inhibiting the raw emortions), so without that being devolped it is hard for him in the first place to control those emotions.
My suggestion, if you can do it, is find a psychiatrist that he likes (that is hugely important), and get a second opinion. (I think ADHD is diagnosed too often, and the medications for that, can actually "energize" any depression he is having.
The other think you HAVE to do to help him control the behaviors he is chosing to engage in, is to set limits and NOT back down an inch. His mother should be doing this as well, but you have no control over that.
I wish I could say this is going to be easy, but I am not going to lie to you. Your husband needs to talk with his ex for the benefit of the child, and if she is unwilling to do so, well, I hate to say it, you may have to have a court step in to get him the help that he needs.
Good luck, and if you do not understand something that I have written, please feel free to contact me at ____@____.com luck.