I really related to this post because I went through a very similar ordeal to your boyfriend. My grandfather raised me as a child and he was basically both parents rolled into one person for me. As a kid he was always there picking me up and taking me to school and even tutored me after school. He spoiled me and took me everywhere and did everything with me. I was going through rough period in my late teens and had dropped out college and was living with him. I woke up one day to hear my grandmother screaming and him laying on the ground choking on his own blood. I was in shock and held him in my arms saying "it's going to be alright" as my girlfriend called the ambulance. But he died in my arms and never got to say his last words because he was choking on the blood coming out of his mouth. After he died it was like my entire world collapsed and I became very depressed and an intense alcoholic and addicted to smoking weed. My girlfriend broke up with me because I was so unmanagable (I would wake up and get blackout drunk every day). I attempted suicide multiple times all while drunk and would often wake up in my bed with a loaded gun in my hand. In the end I went through about 2 years of very intense deep depression with heavy drug and alcohol use. Eventually things got a little better and was able to live with the fact that I'd lost him. It never goes away but time covers the wounds and it got a little easier with each passing day. Several years later I got sober and It forced me to learn some things from back then. My family told me almost a decade later that he had committed suicide by taking two medications that he knew would be a lethal combination when taken together. They purposely didn't tell me because they thought it would drive me even more off the deep end and they were right.

I guess what I'm trying to say here is that there really is nothing that you can do and every person copes with loss and sadness/depression in their own way. If I can give you any advice, if you really love him is to bear with him and stick it out no matter how bad he gets. If you break up with him he may turn up dead by his own hand like I almost did.