Let the psychiatrist and therapist talk him into trying meds. It took years for me to get the right dx and treatment and longer to stay on the meds. I would suggest that you get your own therapist as well as get one for your daughter. Eventually when he's ready you can go in as a couple but right now you're taking care of someone that is sick. Mood swings I view like epilepsy the big emotional outbursts are like a grand mal and the shut downs are the silent type.

You can't judge the relationship while he's sick. You may have to get to know this new person. Even with medications he may be different. I would refrain from trying to label his mental state.

How to survive:
Both be in therapy
Writing down and hand it to the person. It's a lot harder to miss communicate when it's written
Do something together that doesn't involve talking. Like a movie just sit in each other's presence.
Take time for yourself.
Get into the habit of having a date night with your teen. Build up those relationships.