tlm proves your point!!!
FWIW I also changed my forehand, serve and bh all in the last 6 months or less.
The changes are close enough to be confusing, and some are subtle. But it IS possible. I really struggled on my forehand and it was frustrating. But sticking with it I think I am hitting much better form today and it is a more efficient stroke.
I get it as there are some changes my coach asked me to make and I have not really focused on them but I COULD if I chose to. Like the volley. Its pretty ingrained and I would need a few weeks of dedication to change. But I could do it.
Even the things I think are impossible like hitting flat or hitting with an eastern grip could be achieved if I chose to.
I think you are right on the tension. Its all about launch angle. He needs a tight tension because the swing produces a jacked up angle. Your lowering slowly will help him fix that but well we ALL know how that will play out...