If he is just a pre solo there is probably still time to keep him from killing himself. He may also be confused in thinking about this 90 bank you are talking about. He may be looking at the attitude indicator and thinking that the 60 degree point is actually 90. There is some benefit in showing students a 60 degree bank so they can understand how the aircraft reacts when you start to engage in higher bank angles (ie. nose down, lose of altitude, accelerated stall). I personally take all my pre solo students through the demonstration stalls (trim stall, accelerated, secondary, cross controlled) because they all relate to a situation that the student may accidentally put themselves into on a solo flight. 90 degrees seems a little extreme and I assume you would probably stall prior to that if you are trying to do anything but descend.