That doesn't make sense, though. Unless he was trying to get out of the cockpit, he should still be in view. Well, I'm not an expert on Mustangs, but I know a few things about high-performance airplane harnesses. They strap themselves in so tight they can barely move. Even if his head was slumped, I don't think it would completely disappear from view at that angle.
I don't have enough evidence to back up what I'm saying, so just label it speculation and take it for what it's worth.