Documentation and ignoring are all great ideas. As a HR professional, I highly recommend that, at least one time, tell the son that his comments are completely unwelcome. If you can do it front of a witness, then that's even better. Then, document, document, document. The other girl needs to as well. Then, if it doesn't stop, it's time to go to the boss. Once the boss is aware, he is legally required to make a reasonable attempt to stop it. Good luck!