I played a 1H BH so I have a hard time teaching 2H BH. I just sorta hope his teachers get through to him.
I don't think our sons are/were doing what Agassi recommended. I think they are almost using the right hand to lever in the opposite direction.
Another tip that worked for us: focusing on bringing the left shoulder to the chin. Anything to make it a long swing which is less "wristy".
One day, it just clicked and his backhand is pretty strong. Unfortunately he doesn't move like your son so that's a whole other issue.
And the day his BH clicker, his FH went off the rails...