After speaking with Three Dog, head to Rivet City to try and find your father. Head to the Science Lab and speak with Dr. Li to learn that he has yet again moved on. Set off for the Jefferson Memorial and enter the door labeled "Gift Shop." Head to the control room of Project Purity and find Audio Tape 10. Listen to it to find that he has gone to Vault 112. Set off for Vault 112 where a robot will ask you to put on a new outfit and use one of the Tranquility Loungers.
See Scientific Pursuits for more specific info.