Well, I think the MC's head is not blown away.
Look at that explosion (or what it is) and you will see that there is a dark form presenting the half of his head inside.
So my guess is that it is an explosion that "only" burned his skin on the right half of his head.
The parts that seem to be flying away might be the remains of his ear or so I guess.
But that is all.
And that wound probably can be healed with magic by the girl or maybe that strange masked guy will heal it after telling the MC that that explosion was a warning what will happen if he does not show more respect and politeness to the masked guy.