Give him a good soak, and if he refuses to get his head wet, pour a little warm water over the top of his head while he soaks. Keep misting him on the top of his head after that and give him a few days to get it off on his own.
If it still wont come off after that, try using running water and gently rub it a bit around the edges of the stuck on piece with your fingertips to help it along. Work it off rather than yank it off
Hope this helps-
Artemis