Hello and welcome to Yorkie Talk.
He maybe constipated. You can try giving him pumpkin. You can use can or the baby kind also. Cordelia went through this and my vet asked if I had cats and if I gave them Petromalt Hairball Remedy. It acts as a laxative to the dogs and will not hurt them.
http://www.petco.com/assets/product_...847611001B.jpg
Also is his bottom area shaved? If not sometimes try poop will get stuck there and also block poo to come out.