105-110 MPH would do it.
As someone said earlier, 102 MPH is the fastest pitch any player has ever gotten a hit off of. Everyone assumes that a hitter would adjust, but they forget that the pitcher will adjust too. He's not gonna throw right down the center of the plate every time. 105 MPH at the letters is a different pitch and requires a different reaction than 105 MPH at the knees. Changing eye level, adjusting how long he holds it before throwing, etc. Then, if a batter really does seem to figure out the timing, you throw one in on his hands and see if he's willing to set himself up early enough to hit it again after that.