Correct temps and husbandry and then if it doesn't cause extreme stress soaking 20 min a day every day for a week or more (85-6º water) . This will help resolve the dehydration. Often they will not eat if really dehydrated. I would wait a week after that soaking for feeding. If you are feeding other sooner place something the smells of rat near him and if he shows interest feed, light however a weanling or so just to get it started on something small.

About 8 months ago I had 3 snakes come in all with about what you describe (all housed together it was a mess) anyway after 3 soaks and corrected enclosures (yes 3) they all fed aggressively I wasn't planning on it but all three were out almost instantly with the smell of rat. I offered weanlings that I had injected about 2 ml of drinking water (no chlorine) into them (one ml syringe jammed down the through of the thawed (F/T clearly) rat.