get a plastic tub and fill it with enough water to cover the snake's belly but not go over it.
the water should be in the low-mid 80° F range.
put a rough-textured washcloth or towel in there with him and let him be for ~20 minutes.
after that: take the snake out and use the towel to GENTLY, GENTLY start rubbing the snake down and allowing the snake to slither through it. the skin should be soft enough to start coming off. you can GENTLY pull at any loose skin you see to start coming up but BE CAREFUL!