Don't plat tug of war at all with the leash. When he grabs the leash, stop walking, take his collar and his leash so he can't pull and stay until he drops the leash. As soon as he drops it reward with a "Good boy!" and a really good treat. Keep repeating.
While you're woring on this, work on "Drop it" at home. Start a game of tug of war with a rope toy. Pull the rope into your body and hold tight, don't move. As soon as he releases tell him "Good boy!" and reward him with a treat and resuming the game. After he releases consistantly when you pull the toy to your body, start using the "Drop it" command as well. Keep practicing with both the body cue and the verbal command.