That's why he needs a very damp towel over his head...no point soaking his body, he won't submerge his head.
This probably won't be the first time you'll need to help a snake shed, so no time like the present to learn to keep him quiet & cooperating to finish what he started.
Be gentle & persistent, but win the battle of the shed...One reason the old skin often sticks on their heads is that the scales are smaller. Be sure to check the chin-
that's a common place where it sticks & it's hard to get off, even with our help.