It sounds like he has been referred to mental health services for an assessment, you don't get a diagnosis of bipolar disorder from a 10 minute chat with a GP.
It does not sound like he has the symptoms of bipolar disorder from what you say. You've described no manic phase (flight of ideas, talking fast, pressured speech, grandiose ideas, not sleeping much) or symptoms of depression (changes is sleep and appetite, feelings of hopelessness, helplessness, sadness, diurnal variation, lack of motivation etc). I suggest you do some reading around borderline personality disorder/emotionally unstable personality disorder and also narcissistic PD - these are not treatable with medication. There is no excuse for the way he treats you, being nasty is not a mental health condition.
It sounds like he is using a probably non-existent MH problem as an excuse and you're falling for it. Even if he does have MH problems - it is no excuse for the way he is treating you and this does not mean you have to stay with him!
Also I would suggest going to the MH assessment with him or speaking with them separately with his consent. If he doesn't want you to thats concerning - people can say what they want at assessment to get a diagnosis they want, and info from family is always really helpful!! He could come out of the appointment and tell you whatever he wants.
I think you know this is not a viable relationship 