One, he isn't "rail thin" at 6'3" 205... there are a lot of QB's who play at that build. IIRC, JG was 6'4" 195 as a RS Fr when he was being sacked 30 times over a half season because he couldn't get the ball out.
Two, it was a freak fall. It probably would have had the same impact on almost anyone. Have someone hang you by your feet with your head 6' off the ground, blindfold you, then drop you. You'll probably have a concussion and/or neck injury.
Three, it could result in him being more prone to concussions but we have not seen that yet. If I understand correctly, concussions can be cumulative but not always.
Four, there is the possibility that he could be more naturally prone to concussions.... but we don't have proof yet for that either.