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I'm sorry to hear your dog is constipated. I would start treating by adding a small amount of vegetable or mineral oil to the feedings. Just a tsp or less at first (less if small dog, bit more if large breed), as you don't want to overdo it and cause diarrhea. You can always increase the amount a bit. The oil will lubricate the intestinal lining and push the contents along the tract more quickly than normal.
Once regulated, stop the oil, but you may want to add a higher fiber to the diet, like oat bran, regular canned pumpkin, or cottage cheese when starting to see firmer stools, or you can even compare what you are feeding now with higher fiber diets sold in the store (fiber content listed on packages).
If the problem becomes chronic regardless, there could be an underlying medical reason on why the intestines are not absorbing appropriate amounts of water, and then a vet visit with some blood work may be in order. If you ever see increased urinating along with constipation, that would be a key sign that your dog is getting dehydrated and needs vet assistance.
If he's not able to eat due to his discomfort, you could try warm water with a few drops of oil in a syringe squirted directly into the rectum gently to loosen up with is at the bottom-most portion of the colon, but this can be quite messy, and often being blocked up continues up through the large intestine and orally is the best method at home. If it has become too late for this, he will need a professional enema (not at all like the ones we have for ourselves at home) to get him back on track.
Best of luck, and let me know if I can help further. Hope your dog is better in no time.