Guard, high thread attacks, taunt - if he is using all of those and it wouldnt work your DDs are crap. DDs should be able to focus and/or reset their aggro (you can reduce/reset their aggro, too, as a Sith Sorcerer). It is not always the tank. The DDs have to do their part, too. Try to talk to them. Since it is quite easy to get other DDs just kick those who just cant listen.