Hmmm, interesting that most on here react differently depending on whether it's a man or woman suffering from mental health issues.
Just for a bit of balance:
Without doubt he needs to get help. If you still love him, help him, if he won't accept the help and won't accept he has an issue it won't get any better, although I don't agree that it'll necessarily get worse. I you don't love him anymore then it really is time to get out.
However, it is definitely possible for him to change. I did and haven't been a dick like that for almost 5 years. It did therapy (counselling), stopping drinking, exercised and did various other things to get myself out of depression and into a better place mentally, and it was hard work. I honestly cringe when I remember some of the things I did to someone I supposedly cared about. In my case my ex still left despite (or possibly because of) the change (it was definitely two sided, not saying it is in the OP's case of course, but my ex would validate her behaviour because of my reaction to it and it became a bitter cycle of recrimination, when I stopped reacting to her behaviour it made it worse).
But separating made me much happier in the long run as the relationship had run it's course long before and neither of us were really happy and just hanging on to past times (probably what caused a lot of the issues). I'm now in a great relationship and and my mental well being is far, far better.
I would also add that regardless if he gets help or not, you should consider counselling yourself. This honestly isn't meant as a dig at you or saying you have a problem but it is far better to talk out your grievances, the way he acts towards you and the impact it has on your well being etc with someone professional, than with friends or family or on here where people with more often than not just nod, agree and sympathise with you.