Very interesting discussion guys.
Some clarification... he uses a Nittaku Acoustic with Tenergy 05FX on BH & FH, the rubber has been on maybe a month and he plays 8-10 hours a week.
He is forehand dominant but has been working hard on looping against backspin on both FH & BH.
I've been thinking that on the BH the timing is a little off, i.e. it looks like he is reaching for the ball, not waiting enough... wondering whether as a result the bat angle is not perpendicular to the incoming ball direction, hence the top of the bat is further away from his body than the handle and that this is causing the impact pattern ?
I will relook at the pattern tomorrow when I get my hands on the bat again and he's used it a week longer to see if anything has changed.