Your best option might be a vet exam with perhaps an x-ray or ultrasound to see if there is any obstruction or partial obstruction along his digestive tract.
Sometimes items move slowly along until they get to a point they get stuck and cause a blockage or even a twist or telescoping of the intestinal tract.
With the symptoms you are listing if you can see your vet with him tomorrow you might want to do that.
If he isn't passing stool or is only passing a little or diarrhea that might be a sign of a partial or full blockage.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics