To my knowledge pecans are not toxic to dogs but I certainly would not be feeding him a whole bunch of them. I would be concerned that they would cause gastrointestinal problems like vomiting and diarrhea. Below is a website that lists most of the foods that are toxic to dogs. Macadamia nuts are definitely toxic so please make sure you don't feed any of them to him.
Please let me know if you have any questions.
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