In Fallout 3, I lost my bodyguard, Fawkes, when I completed the main storyline. Where can I find him?

I made him wait outside the purifier chamber while I activated it and I past out. I woke up in the citadel and I have no Idea where to find him. I check Jefferson memorial but I still can't quite find him. Any help? Oh, and when I was doing a DLC, it gave a notice that Fawkes was returned somewhere and was waiting for me after I finish the DLC. The problem is...Idr what it said on that notice..