A few things you could try......
Put him in a rubbermaid with a damp towel overnight.
Put him in a rubbermaid, with an inch of luke warm water, overnight.
Just keep misting him directly once or twice a day.
Double check your husbandry. Your ambient humidity may be too low. It also helps to mist the snake daily, from the time they go blue until they shed. Are you by chance keeping him in an aquarium with screen top?? You might want to look into a rubbermaid enclosure.
Hope this helps.
Colin