He doesn't bother to flank, which doesn't mean that he doesn't get the bonus.
But let me quote the sneak attack skill:
Uses the rogue's ability to approach unseen, adding additional bonus damage to their attacks when the target is Blinded, Flanked, Hobbled, Paralyzed, Petrified, Prone, Stuck, Stunned or Weakened, as well as when any target is struck within two seconds of combat starting.
So that is a LOT of status effects. Most easily accomplished by placing the HUGE Druid hobble spell infront of your party. In addition throw a flaking spell from a Cipher, which is huge as well. It's extremely easy to get the bonus without actually positioning your rogue in dangerous places.
That said: if you position him in the middle of the enemies, he will die a very fast death anyways. Not but your casts but from the enemies beating on him.