It's the shed.
Soak him in a ever-so-slightly-warmer-than-room-temp bathtub. (80 degrees is good, if you have an IR thermometer.) Make him swim in a couple inches of water for a good 5-10 minutes. At that point you can peel his neck by hand. He may even like it, once he figures out you're not just poking at his head to be a meanie.
That'll cure the wrinkles. But next time he's in shed, you'll want to find some way of boosting the humidity.
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