Well, alright, there's also through the bottom of the seat, up through your feet, into your back from below...
Though the last is the least likely due to seat armor.
The P-38 was actually one of the most susceptible to pilot wounds. They added an armor plate behind the guns to try to reduce the number of injuries to the pilot.
EDIT: Basically, if he has a film he can send it to HTC and they can extract the exact trajectory, velocity, and penetration of the round to see if the bug exists or if he was shot in the butt. It isn't all that big of a deal, though.