I've also just seen your post. Sorry that you are in this situation now.
Having also been though depression and medication and the battles, it's a difficult journey, however with a lot of time and effort I have been off medication and no depression symptoms, so the black dog can go away, but only time will tell if it's gone for good. So it's unknown how your partner will go.
There is often a second battle fought by the partner, (you) and it's just as hard.
As Paul says, read the info on this site, it's very helpful. But there is no quick fix. It's about being supportive but also looking after yourself. As it can be quite draining having someone close with depression.
Getting professional help is really important, and unfortunately some professionals are not as good as others, so someone needs to check that he has the right person helping him. Bases on your early comments about his initial traits I got the impressions that he may have some issues from his childhood haunting him, so that could be underlying his condition. But that's what a professional should be looking for.
His recent negative behaviour change, sounded to me like drugs may have been involved, stealing, etc. was the the cause or the outcome is unknown. Him hurting you may also have been a sign that he cared, and thought pushing you away would be best for you. Sometimes the more painful way it's done could be to stop you coming back to him, and therefore protecting you.
Sounds like there is a lot more to his story than you may be aware of, and he may not be able to share it with you. I would think that he is not out of your life and that he will return again. As others mentioned, it can be quite painful so you need to decide where you want to be. And I would suggest that you take the controlling role, you decide on what terms the relationship continues and what he must do.
But please do it with love, he probably needs a supportive and firm guide, not a slave driver.