From the Fallout Wikia under the entry for Fawkes:

When the player is faced with the climactic decision at the conclusion of the game, Fawkes politely declines to enter the radioactive room in lieu of the player (even though he is immune to radioactivity), stating that the decision is the player's destiny and not his. However, if the player has the Broken Steel add-on installed, the option to send Fawkes in will appear, allowing the player to opt out of entering themselves or Sentinel Sarah Lyons. When this option is used, he states that although he was going to tell the player that the player's destiny lies in that chamber, he has already altered his own.

Yeah, so prior to Broken Steel, this option wasn't available. Phew! I was starting to thinks my character and Fawkes just weren't as tight as Xhonzi's character and Fawkes... and that made me sad.

Also googled the FEV stuff, wow, didn't realize it did all of that. That is really cool, kind of wish I would have now.