Get a small plastic tub with a lid that will hold water, put about an inch of lukewarm water in, as well as some paper towels to make it soppy and add some friction. Put the snake in the tub, put the lid on, put the whole thing on the warm side of the snake's enclosure, and leave it alone for an hour. This creates a nice snake sauna and the skin should be fully off. If not, wait a day and do that again. Letting him slither through a warm wet rough towel will help too.