Kind of a dumb question but it has to be asked. Do you know for sure if he has blood vs frost on? Sometimes DK's who occasionally tank forget to turn off their frost and this can cause plenty of havoc.
Also he should never be using Death and Decay to do AoE damage as this is actually a high threat tanking ability. Look for the red boiling aoe they cast. Only DK tanks should be using it not a dps. If he is using this tell him to stop along with double checking against frost aura being up.
Other than that I would just say to make sure everyone is using a good threat meter and work on your rotation and ability to check threat against all the mobs you are tanking. The days of simply casting consecrate and standing back are over. You need to cycle your targets and make sure you are getting the most out of your hammer and shield.
DK's sometimes are annoying in that they are obviously OP and some think they are all that by pulling threat on purpose. Let them understand that you are doing your job tanking and if you don't get hit you can't get mana. Not to mention I'm sure your healers would appreciate knowing who to heal instead of having to constantly worry about the dps DK who suddenly takes damage all the time.
_________________
