Hi,
From the sound of things it sounds to me like you are a mother to a bipolar son (right?) Most of the time when I am having an episode I realize that the only thing keeping me alive is my daughter and although it seems 100% true, we with bipolar, our emotions deceive us and I'm not sure if that is true of your son or he just really believes it because that's what his emotions are telling him. (His emotions, I think, are just basically lying to him sometimes, remember). If he's told you he lives for you, be happy. He loves you that much and it is giving him a reason to continue (and does the reason really matter as long as he keeps at it? You are his light when it's so dark he cannot see, be grateful of your strong connection).
As for the sociopath bit, from my experience, sometimes, in my bipolar, I flash to angry so fast and I don't care who gets in my way, who I hurt. Although during I do not, afterwards I am a wreck and feel such guilt. The sociopathy he feels is probably during his angry times, as if he truly has a gentle heart he could never be a sociopath, trust me. I really believe us bipolars are so, so, empathetic that sometimes we shut down as a coping or survival method (I personally have never been able to do this, but I don't doubt your son has or can).
I don't know how you can help him without getting pulled in, I struggle with that with my friends and family too. But I do know that I often do not say anything out of fear of pulling them down a lot. Maybe you could ask him to share his day or feelings/emotions after he has gone through an episode and is calmer so that it doesn't overwhelm you too much.
Hold on! He may not see things now, but with bipolar our emotions are always changing so hang in there and pray for him and things will look up, bipolar really is a cycle....