Here is a good, short blurb on bipolar
that perhaps your fiance will read and understand it a bit more. That might help the situation some.
As far as the three of you go, I would sit down with a family counselor and talk this one out. You may "let" you son go farther than he really needs to go, and your son may be testing the waters on how far he's going to be allowed to go. This would all be very normal for a newly forming family, and a teenager as well.
You son needs to understand that neither bipolar or ADD
gives him any more rights at 17 than anyone else has. If he can control himself out in the world, he can do it at home, and needs to learn both his own controls and other's boundaries. If he is so incredibly out of control that he doesn't know what he's saying or doing, then outside intervention is called for.
I've worked with adolescents my entire career, and I can tell you one thing: very few are out of control in the real sense of the word. I've seen them act this way in front of the parents and then snip it right up as soon as they leave. I've seen them stand down in the face of consequences they choose not to lose..when it's someone besides the parents in authority. This means that most have the inner control...they just don't use it when they don't have to. Your son has to step up to the plate and take charge of handling his illnesses as he's on the verge of adulthood, and no judge takes "I'm bipolar with ADD" into account when sentencing. It's an incredibly harsh truth, and you might be the only one to help him really understand it.
As far as the fiance, he has to come to grips with his lack of control in this situation and find where his true power lies. Butt whoopins don't cure mental illness. Sitting down and talking about the truth of the matter and helping him understand that your son's illness isn't something that you would wish upon anyone. You son isn't responsible for having ADD or bipolar, but he is accountable for his actions in all situations unless he is in a state of medical emergency and needs to be hospitalized to protect himself and others. Short of that, it's something to be discussed and worked through.
As things cool down, hopefully the extremes will get taken off the table, but I do recommend a good discussion between the three of you with a counselor, and if not, then alone, with each person speaking for themselves, one at a time.
My best to you in this very difficult situation,