Just a quick question. Where dose he go when he's fired? and if he's not their how do I find him?

Never mind a not just popped up that he's returned to the "ninth circle".......took a while.

All Companions usually return to where they were originally found.
Jericho returns to Megaton.
Paladin Starr to the Citadel.
Dogmeat to Vault 101
Charon to the Ninth Circle.
Fawkes to the entrance of Underworld.
RL-3 to Cantobury Commons - stands next to a bush on the outskirts
Clover to Paradise Falls.

If I missed any here's the link if more info.
http://fallout.wikia.com/wiki/Fallout_3_companions#Firing_permanent_companions

Anytime you want to fire a follower, it's best to walk them back to their default firing location first and fire them there. If you fire them anywhere else, it could take them many months to get back to where they need to go and they can even die along the way. So if it's someone you think you might want to use again in the future, best to take them back to their default location and then tell them to hit the bricks.