I don't think he disrobed because of hypothermia. Something else was going on, perhaps a head injury, but it's hard to say looking at the extent of damage to the vehicle.
I've been in an accident similar to his, but it wasn't on a gravel road, it was whiteout conditions in the middle of a winter storm. The car spun around, left the road, went into a ditch after hitting a fence and ended up facing the opposite direction. My husband and I just sat there for a while getting our bearings and realizing we were just shaken up, not injured. We couldn't get out of the car until the cops arrived with a tow truck.
I wonder if the same type of thing happened to Jason. Maybe he couldn't get out of the driver's door since it was up against the fence so he crawled over the console and left through the passenger door.
Whatever happened to Jason, he had the wherewithal to collect all his belongings, including his fish, and get it all out onto the road. That's when things get a little muddy.
His clothing was found about 900 feet from the vehicle. I can't see anyone suffering from paradoxical undressing during the short period it would have taken him to walk that distance. It wasn't cold enough for that to happen. Even if he'd been stunned for a while, sitting in a cooling vehicle before he got out, he still wouldn't have been vulnerable to hypothermia, IMO.
I don't see any evidence that someone was with him in the vehicle. I suppose there is a long shot that someone came along before the VFF and took JL but why and why leave all his belongings including his wallet with money and ID just sitting there in the middle of the road?