My dog is constipated; changing to wheat free dog food; have done it gradually as told; today he is struggling with bowel movement; what should I do?

There are many causes of constipation. He could have swallowed something that is obstructing his colon. Maybe try adding some fiber to his diet (i.e. fiber supplement, bran cereal). If he has been constipated for more than a few days, consult your vet.

If you have changed his food suddenly then this is why.

Dogs can't handle a sudden food change, you need to gradually introduce the new food day by day until they are only eating the new one.