Make sure his humidity is 50-60%.

He could be about to shed. But if he really is dehydrated, give him a soak in warm water about 1/2 as deep as your snake is thick in a NON-airtight container - set it inside his enclosure to keep the water warm. 20 - 30 minutes should be plenty. Do it now and in a few days and see if he improves. If not, take him to a vet.

Good luck!
Melissa