How do you get Fawkes even when you already have charon in Fallout 3?

""WARNING SPOILERS""

The only way you can get fawks is to find him in one of the main quests. The one where you try to recover the G.E.C.K. from the vault. He is trapped in the vault and will ask you to save him. You may find it nearly impossible to procede anyway if you don't save him. You will encounter him later after you escape from Raven Rock(ASSUMING YOU SAVED HIM) and he will talk to you. After that you will need good karma for him to want to help you.

also if you had him as a follower but lost him. (E.G going to pointlookout meaning he has to be left behind) you can find him in the museum of history. he will be walking around inside.