After dealing with the same sort of problems off and on hydration is critical soak him for 20 min 2 a day for two days then place him in a enclosure and make sure the husbandry is perfect (70% humidity however) and leave him be for at least 10 days try not to mess with him at all. I'd then try a mouse pink they are tiny and not a scary food item. Personally i'd avoid assist feed if at all possible. The stress the animal is under and the potential dehydration could lead to a re-gurge that in its current condition it may never recover from. The last re-had snake I got was 10 inch and 31gm and in seriously poor shape (stuck shed and mites) getting properly hydrated with the soaks makes an unbelievable difference.

Good Luck