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If you think that you overdosed him, you should see your vet right away so that the vet can give activated charcoal and other supportive measures.
If you JUST gave it to him within the last hour, you might be able to bring it back up by giving him 2-3 tbsp of hydrogen peroxide by mouth. This will force a vomit.
If you can't get it back up, you really need to see the vet.
Let me know if you still have questions.