How are his stools? Are they on the more solid or hard side? You might try adding a little canned pumpkin to his food (not the pie filling). There could be other causes that your Vet would have to examine for.
Margot gave excellent advice about feeding him the canned pumpkin. You might also speak with your vet about giving him a stool softener. It could be that his stool is hard enough to hurt when he tries to pass it. A stool softener would alleviate that. If these two things don't work, he needs to be seen by the vet to make sure it's not something structural that is giving him a problem.
It knd of makes sense because with both dogs of mine I noticed that their first poop of the day (usually after a night's sleep) was firmer than the 2nd or 3rd one.
Also, what shape is it when it comes out? Normal, or slightly 'squashed-looking'?
Even if it's normal, I would get the vet to check out his Prostate gland. It is easy and quick to do and needs no anesthetic, to check if there is any basic swelling. But this could also be done by X ray, for more detailed results.
If his poop is slightly squashed-looking, then I would suspect a Prostate enlargement.