What to do if your puppy is constipated?

Feed him bran muffins. Or a can of salmon. Or both. It should help By tomorrow morning. If he's in a lot of pain, you can try giving him an enema. Depending on the size and temperment of the dog that may be quite difficult.

ANSWER
I would suggest you take him to a vet or animal emergency hospital. Do not attempt to give him an enema. There may be intestinal blockage that needs to be surgically treated.