When I knew the family he was still using his Response, weighted and gripped to his preferences. I never heard anything about the flex being customised - could well have been. He wasn't much of a gearhead from what I can tell.
That was just after his second US Open win. I have no idea if he actually changed after that or just kept paintjobbing.
The more recent pics certainly look like a Yonex though.
"A closed mouth gathers no feet"