Broken engagement...now he's calling?

I dated a guy almost two years ago. I fell in love with him, but he did not feel the same way. We have steadily worked on becoming friends, and although he became engaged in the Spring, we both decided that being friends would be a priority in our lives...though the idea of him marrying someone else has been very painful for me, and I had secretly hoped it would not work; beyond my own feelings, I never thought she was right for him. Last week, she broke off the engagement. He is devastated. He says that I am the only person he trusts and can really talk to. I want to be there for him, but I am worried about my own heart, too. Any advice?