Here is some great information on constipation. I hope it helps:
Ask the Veterinarian: Dog constipation, psyllium seed husks, oat bran
Make sure you have checked his bottom to make sure he is not have any caked feces that would prevent him from pooping. The pure canned pumpkin works great for constipation and diarrhea. You would give him 1 teaspoon morning and evening. Please keep us posted on how he is doing.