While this drug is not usually used in veterinary medicine, dogs can rebound within 4-12 hours. Signs you will likely see with ingestion could be weakness, drooling, hyperexciteability, panting, disorientation, etc. Here is a link to an abstract studying this exact drug in dogs.
What is important to note with this study is that no deaths were reported.
If he did get one, you can try to induce vomiting with some oral hydrogen peroxide. This may get the pill up and help ease your mind. I hope this helps and gives you direction.