Are you on PC or console? He's supposed to be essential, so I'm not sure how he would have gotten killed, unless there is some kind of non-essential mod.
If you're on PC, look at him and then open the console with the ~ key. Type resurrect and close console with the same ~ key. This should make him disappear for a second, and then reappear alive. If you type resurrect 1 instead, then he should just stand up.
If you're on xbox or ps3, I have no idea how to help, other than to try to load a previous save and see if he's still alive. Make sure you save first, though, so you don't lose all your progress. In case he's still dead, at least you can come back.