They definitely do fall asleep back there. I always knew when my daughter was sleeping because her helmet would hit the back of mine as she rocked back and forth.
With the big Givi top case and two huge Givi side cases on my V-Strom never once worried about her falling off.
She would have to climb her way off. Without the cases it was a different story. When the cases were off, she was more nervous all around. Kept her awake, and eventually off the bike.
I also had the benefit of my wife riding behind us and being in contact over the bluetooth. Not that it was ever necessary.
Get big cases. They make everyone feel better, and they are safer. For the cases you are showing you might consider strapping a couple of camping bed rolls to the top until you get a feel for how he's going to react.