I think his dream is probably about him, not you. It is a little creepy that he happened to have his dream and call you when he did!

Your dreaming about your brother dying could have something to do with his doctor/cholesterol conversation or it could be about a "male" part of you; maybe still about your boyfriend and his graduating and not seeming to be "functioning" well?

I think that it is your brother, just shows the closeness of the situation to you (can't get a whole lot closer, unless you dreamed you were a twin but, since I don't think you are, that might not be something your unconscious would do?), like when we dream about mothers, fathers, etc. Or it could be about someone, a male coworker, who is a brother-type orientation to you.

I don't know your relationship history with your father, either. Could be, if your father is unavailable that a brother is taking the place in your psyche, kind of a "Benny and Joon" thing? Is your brother older or younger? Don't forget "big brother" watching; could be about new regulations you don't like or that will interfere/"kill" your dream, etc.