Purina is a very poor kibble, therefore if you're going to go for a higher quality kibble like Origin, here are some natural ingredients you can add to your dog's kibble to help with constipation;
-Make sure your dogs always has fresh water available (This will help loosen stool)
-Make sure your dog gets plenty of exercise and chances to poop (If he has to hold it too often, he may develop more constipation problems)
-Add Psyllium or Metamucil which are fiber supplements to his diet (Half a teaspoon twice a day is what I'd start with, and make sure to only use this with canned food and or plenty of water) *Remember, this method used as an enema should only be used occasionally or it may lead to obstructions in the bowel.
-Add mineral oil to your dogs diet which will loosen stool. For dogs less than 10 lb., use no more than ½ tsp. If the dog weighs between 10 and 25 lb., use no more than 1 tsp. Medium-sized dogs weighing 25 to 50 lb. should ingest no more that 1 tbsp. per day. Give no more than 2 tbsp. a day for large dogs over 50 lb. If all else fails, you can put the mineral oil in a syringe, and use it as an enema.
-Canned pumpkin (As a rule of thumb a couple of teaspoons daily for a small dog or a couple of tablespoons for a large dog)
-You could try adding some bran to the food (Bran is the indigestible outer husk of wheat, rice, oats and other cereal grains) Dogs can't digest grains real well at all, so this could potentially help to form more loose stools in the case of constipation.
-I have found that fatty acids like fish oil and olive oil are very helpful. You can give dogs the same fish oil that you take by the capsule is you wish. Just keep your dogs size in mind, and following the serving size recommendations on the back of the bottle. You can also purchase actual oil in liquid form made for dogs. Just follow the label. If you decide to give your dog some olive or vegetable oil, another good rule of thumb is to give a small dog somewhere around a teaspoon, and a large dog more like a tablespoon. Just give it a shot and see what works in your case.
-Flaxseed (Just like the oil, try a teaspoon for a smaller dog, and a tablespoon for a larger one. Se what works for your dog's particular case)
-And you can also try adding any other fiber enriched foods to your dogs diet. Just make sure your dog has access to plenty of fresh water to assist with the wonders of the fiber.
Hope I could help! Good luck!:smile: