How can I be there for him while keeping my feelings in line?

I got a text message at work with my boyfriend telling me that Aunt Jaunita (whom I have met) has cancer and it has spread to her liver. And the doctor isn't sure if its one type or two types of cancer. Surgery isn't an option because she has congestive heart failure. So the doctor isn't sure if he can treat it or not. I lost my Aunt to cancer last year, so I know how it feels to have someone who is or might die of cancer. And what makes it worse is that when I'm around his family, I feel like part of the family. I wanna be there for him, but I didn't really have anyone there for me when my aunt died. What can I do to assure him that I know what he's going through and will always be there for him?