For safety check to make sure there is no dried poop in the hair around the anus as this can cause the puppy to delay defecation because it is uncomfortable. If there is dried poop in this area you can gently bathe it away with warm water and a little mild dish soap or if you have a small hair trimmer you can clip it out. Using scissors for this should be avoided as it is very easy to injure your puppy.
If you see no dried poop or inflammation of the anal area he may be truly constipated. I can give you instructions on trying to resolve that but I would need to know his weight.