Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern about Cooper but, fortunately, he's not likely to develop any significant issues as a result of ingestion of 5 mg Zolpidem...primarily because he's such a big dog.
Mild sedation might be seen at around 0.2 mg/kg which calculates out to be about 9 mg for a dog this size and clearly he only ingested 1/2 this amount.
I wouldn't expect him to display any symptoms but if he does, they should be relatively mild.
Signs which have been reported include balance issues hyperactivity, vomiting and lethargy; panting, disorientation, nonspecific behavior disorder, and hypersalivation as well as increased heart rate, tremors, apprehension, vocalization have also been seen at higher doses.
Signs typically developed within 1 hour and usually resolved within 12 hours just to give you an idea of the timeline.
More than likely, however, he'll be just fine.
I hope this helps. Deb