How do I stop my son's songs from syncing to my phone?

Use "Settings > iTunes and App Store > Automatic Downloads > Music" and turn this OFF.

When you sync your phone to the computer make sure to deselect Sync all Music or deselect the playlists you do not want.

Also see https://discussions.apple.com/thread/3958277?answerId=18409815022#18409815022&ac_cid=ha

And Here... http://support.apple.com/kb/HT1495

Char1285 wrote:

My son and I share an account

You may also find the information.. Here-of Interest

Thank you! This is the answer I have been searching for.