I'd just go to some area where followers will automatically leave you. If you have any of the add-ons, those are all solo-areas, and if you go to them your follower will be fired. That should cause him to go back to Underworld.
You cant drop charons contract its a quest item. he probably still is at vault 112. if you have not done the quest yet then SPOILERS
get arrested by the enclave during the waters of life quest. you will get a message your followers were fired. in which case means they will return to their recruitment site. except for dogmeat, he will be waiting @ 101.
Krauser: " Whats worse is... he's doing your mom too..."