Give him a warm bath in epsom salts. No drinking since it is a laxative. Try using a rubber glove with some oil or lubricant, and inserting a finger into the vent an inch or two to feel for hard droppings or break them up.
Is he drinking normally? It might be constipation from not drinking enough. You can mix 1 ounce of molasses into a quart/liter of warm water, and feed it to him for 4 hours for a laxative. I would make sure that he gets probiotics or some plain yogurt if his feed doesn't contain them. Purina and...