Simply falling on this switch wouldnt cause it to change positions - it requires a deliberate act to do so, the switch requires a certain force to pull up and then move to one position or another, its not like accidentally changing channels on your TV because you sat on the remote.
Also, there is no button or switch he could have fallen on which would have caused the gradual descent that we know the aircraft took. Changing the auto pilot altimeter requires you to use a dial and then confirm the change in two separate actions. Any interaction with the side stick would require the auto pilot to be off, which would mean we should have seen a lot of other, large movements in the aircrafts path, which are completely missing from the telemetry we have at the moment.
The few commands that we see in the telemetry (and by telemetry I mean the transponder tracks, which cover speed, height and directional changes) indicate that the aircraft was under either the control of the pilot or the autopilot for the entire duration of the descent.