OK, I have a friend visiting at the moment and I just walked out in the living room and asked him to just extend his arm. Then I pushed down on it in the approved manner. His arm bent. I followed your directions, so I don't know what went wrong. You got any ideas why that description didn't work?
Have him open his mouth when he does it and check very carefully for how he is holding his tongue. Also, make sure he's lifting one of his big toes.
Seriously, I'd tell him to put more attention to really extending his arm, as if he's pushing against a wall behind your shoulder. That has worked for me against some humongous guys and that is the essence of what I do.