The explanation of adrenaline not kicking in or BG being too low for it to do its thing sounds good. There are devices you can buy that are like watches that measure your BG values during sleep from the skin responses. I would imagine that the manufacturer or doctor dispensing such devices would have a lot of information about negligible sweating during a hypoglycemic, sleep-time attack.
I'm not a doc, just the mom of a teen type 1, so don't take this as formal medical advice. But it is not unusual for the body to sometimes be fooled by a low blood sugar, especially if it drops very rapidly and goes below the 'usual' low range. If he normally experiences lows in the 30s to 50s and gets sweaty and shaky then, but if his sugar dropped quickly below those levels into the teens, his adrenalin might not have even had time to kick in (that's what leads to the shaky, sweaty, fight-or-flight reaction).
He is lucky you were there to help him! If it happens again, he should talk to his doctor about it, and perhaps they'll tinker with his insulin/testing regimen.