Confine him for a while in a ventilated 'critter cottage' with a very damp towel- he may work it off himself, but if not, it should soften with increased humidity & it's up
to you to help him get it off. Gently restrain his head, be both gentle & firm, & above all persistent. Lack of photo doesn't help, but sounds like the old skin is just a
little stained, but it does NEED to come off. Be sure you always check that eye-caps & tail tips shed properly too. Might be time to re-evaluate the humidity in his
enclosure, & maybe offer a humid hide (midway between warm & cool ends of cage) & be sure to check it every few days so it doesn't dry out.