Syncing iPod and iPad
I gave my son my iPad which has a large library of music from iTunes. How can he sync his iPod with his library to this iPad without losing from either device?
iPad
He can't "sync" or he will erase everything that is already on the iPad - if the music was purchased with your ID and if he has a different Apple ID than you have.
Read this. He may be able to drag his music onto the iPad from his own iTunes library by Manually managing the content.