I have to agree with vashti on this one. It sounds like manipulation. Sure he has emotional issues, but he is using your willingness to help him as time together. A sort of screwed up date if you will. Why do you think he told you that he has feelings for you? Why do you think that he got upset and stopped answering your calls when you told him you were dating his friend?
You are allowing him to involve you in his mess of a life and are also now part of the reason he is depressed. Step back and let his family and the professionals handle this. If he is better in the future maybe you can be friends again, but you also have to ask yourself how being in contact with him will affect his depression since he has feelings for you while you have none for him.
Your compassion is a bit misguided here and you involvement is actually a hindrance to his recovery since he has feelings for you. I'm sure that isn't what you wanted to hear but it is what it is.