I Am In Need of Some Advice

Hi guys. So, I've had this friend River for years now. He's always liked me, and I never actually dated him but I did like him in the past. I'm at a point in my life where I've really moved on and I'm in a happy relationship currently with someone else. I'm not interested in River, but here's the issue: River suffers from extreme depression. He doesn't answer the phone for days on end, and I just heard from him a month later, and when he does he speaks of how he's cutting or how he's held a gun to his head countless times. A lot of emptiness. Darkness. It really makes me sad. But, River has fully admitted many times before that he's in love with me. But...I am not in love with him. He doesn't even know about my most recent relationship. I don't know how to tell him without being nervous about the reaction. I don't want him to do something with a bad result (im not in any way leading him on). How do I help my friend most importantly, and two how do I tell him I'm taken? Thanks guys.