Personally I'd step in & help: if it's only his head, he will stay calmer by just putting a damp cloth over his head for a while (rather than soaking the whole snake).
Then after a while you should be able to help him (gently!) remove what's left, or encourage him to slither thru a damp towel (on which you put a little pressure to
help the old skin come off in the towel as he moves forward).
In my experience the skin left on face, chin & eyes only gets harder to remove in time...so I don't wait too long to help. Be sure to check tail tip too, always.