The burns look superficial, but over a large area. It is possible that he did die burning, from heat stroke, or maybe from inhaling heated air, but not from his flesh being burnt. To be honest, I've had much worse burns and totally healed except for some barely visible burn scars, though not covering my whole body. He could also be the victim of a flash burn explosion, that could through such a shock that he had multiple internal injuries.
Look at his head? That looks like something hit him hard, like an explosive fireball.
Fireboots