He won't move. Just run over there and drop it on him. Easiest for PT's because they can still hit him, but when it happens to me I still try to use wrecklessness+force lightning and a pull to keep some hate up, because if your DPS are going balls to the wall he will likely turn for them when the phase is over.
What we have noticed in the last few weeks is that it doesn't matter what you do as a tank. The last rock throw will be an agro wipe and he will turn to whichever person hits him 1st, usually a dot class. We tested it several times in a row, and each time Zorn threw the rock, if a DPS hit him with an ability, or a dot tick before the tank, he would instantly turn to a dps and the tank has to AE taunt to get it back to prevent fearful. Immediately after is a leap as well.
Anyone else notice this?