It may be that he has eaten something that has caused him to be constipated. It may also be something more serious such as a blockage somewhere in his digestive tract. I would not let this go on too much longer before taking the animal to the vet.
My Adult Yorkie Hasn't Had A Bowel Movement And This Is His Second Day. He Has Eaten But Not As Much As Usual. Still Taking Water. Should I Be Concerned?
Yes - could be a blockage. Please take your dog to the vet.