Dear friend,
It is always concerning when a dog cannot pass stool. The first thing that comes to mind is a blockage. If he had any rawhide, greenies or ate grass any of those could cause a problem. You can give him some cows milk as most dogs are lactose intolerant.. That should produce a BM in and hour or so. Try to tempt him with cottage cheese or boiled chicken. If he refuses to eat and cannot defecate even after the milk you really neeed to have him seen by a vet. Imacted stool can cause an infection and poison his system so it is a very serious thing to not be able to move it through.
Please let me know how he acts after the milk and if he goes or not. I will stand by to hear from you.
Very sincerely,
Terri