I know you don't want to leave in fear it will make him even more depressed (I say this because of one of your comments)... but let me tell you a little story.
I was dating this girl for about 6 months when I decided I was done with the relationship, she had very chronic depression though, I was worried I'd hurt her so I stayed. I stayed for about 2 more months, but you could just tell I wasn't mentally in the relationship. I finally decided to end it with her and I got the response, "I hate you I am going to kill myself all because of you."
So, I asked my mom to drive me to her house and it turned out, she really was trying to kill herself, she had taken a bottle of ibuprofen and half a bottle of vodka. She was taken to the hospital where she had her stomach pumped and stayed for about a week.
We didn't talk for a really long time after that. She felt horrible about making it seem like it was all my fault, and I felt relentless guilt on doing this to her. After we finally started talking again she told me she was sorry for making it about me and I apologized as well. This happened 5 years ago and SHE is now a HE and living his fucking best life.
What I hope to get across with this story, is staying isn't the best thing to do. It sounds like you are already checked out of the relationship just like I was. If I could go back and do it over I would have ended it way sooner just to save us both the months of sadness and guilt.
(p.s. in case it matters I am also a female)