You say that he has tried to "take his life twice" in the past 2 months; what did he do or take and, given that it appears no emergency services/doctors etc were called, how did it come about that he didn't succeed? Did he take an overdose and were you able to get to him in time and revive him - if so, did he tell you of his intentions?
It is unrealistic to expect that you can babysit or police this man throughout all of the hours he isn't at work and there is a vast difference between 'being there' for someone who is endeavouring to help themselves by seeking medical advice to cope with depression, and being the virtual prisoner of someone who not only has no intention of getting outside help, but is also using you as the whipping girl for their moods.
If you allow yourself to become the victim of emotional blackmail of this nature you will find that your own mental and physical health will suffer to the extent that you won't be able to think clearly and may become too scared to leave him for any length of time - which, it would seem, has begun to happen,.
Regardless of whatever preventative measures are put into place, the stark truth is that if someone is determined to commit suicide they will find a way to do it and, notwithstanding the fact that the balance of their mind may be disturbed at the time and/or they may have acted entirely out of character, the ultimate responsibility for the deed lies solely and wholly with them.
You didn't create his problems and, as it cannot be said that you've added to them in any way, you ARE free to leave him to his own devices but I would suggest that, before doing so, you alert his GP, those of his family members who are not unwell, and any particular friends/colleagues he may have who may be willing to attempt to persuade him to do what so far he has refused to do for you and get medication that is specifically designed to alleviate depression.