Be sure and read the important note below before trying any of these methods. If your dog is experiencing constipation on a regular basis you should add roughage to his diet such as canned pumpkin (not pie filling kind) or Metamucil. A spoon full of pumpkin on the dog's food should keep his bowels hydrated and his feces soft enough to pass.
A spoon of mineral oil on his food can also be used but should not be used all the time.
There are two methods I use for young puppies, which may work on your dog. I first try lubricating a rectal thermometer and take the puppies temperature. This frequently stimulates the puppy into going. If this fails and you have a nasal bulb syringe (aspirator), you can put warm water in it and gently squeeze a small amount into the rectum and see if this doesn't help.
You can use a children’s glycerin suppository depending on the size of your dog but the other methods are what I recommend you try first.
IMPORTANT NOTE If your dog ate a non-food item, bone or rawhide recently or if your dog is experiencing other symptoms such as vomiting, not eating or drinking, severe abdominal pain, the dog needs to see a vet. The above signs could indicate an intestinal obstruction, which you can read about here:
if your dog becomes dehydrated. You can tell by pulling up the skin between his shoulder blades and seeing if it flows back down normally. If it stays tented or is slow to return to normal, your dog is dehydrated and needs fluids immediately and should see a vet as soon as possible.
If gums and tongue become pale and not a normal pink color or the rectal temperature is over 103F (101-102F is normal) or your dog becomes lethargic and non-responsive, your dog should also see a vet as soon as possible.
I hope you find this information helpful.