That 90 minutes number is interesting.
I'm a sleep walker and it always happens right around the 90 minute mark after going to sleep. I assume it has something to do with sleep cycles, or whatever.
I fell out of my bunkbed when I was young, too. Mine had pullout drawers along the bottom that I fell onto from about 6' in the air. I landed on my back. Never woke up from my sleep. I had bruises all over my back.
I have found (waking up during the "dream") that I am very aware of my surroundings when I'm sleep walking. Your son probably is too, he probably just hadn't had enough time to acclimate himself to the new bed yet.
When I am sleeping anywhere but my own house, I leave the lights on so that when I wake up, I can figure out where I am fairly quickly.