buy his gear, it would sound like he's into lightweight backpacking, but I would guess he either A) has had some experience using his gear, or B) has just read a lot and assumes his choices will work.

regarding him being found at mile 21, it isn't in a particularly tough section of the trail, however, the Salt Fork Creek crossing is at mile 20.2. while most of the time it is a dry crossing, it does have the reputation of being flooded on occasion and can be dangerous. it is very possible he got wet here and was trying to dry off at camp a little ways down trail.

knowing how the weather can be along the OHT, if he was wet, outside of his bag, and the temperature dropped quickly, he could have easily been susceptible to hypothermia.