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If the coughing/hacking started immediately following ingestion of the treat, and he is still continuing to do it, I consider that well worth an immediate veterinary examination.
If, by chance, it is lodged in his esophagus, you can try giving him some water or any fluid he is willing to drink to move it down.
Now, if it is not in his esophagus and it was possibly accidentally inhaled down his trachea, that would still warrant the more immediate veterinary examination and possibly a side view x-ray of his neck to see if the treat is visible and truly stuck.
Tough situation but hope that information helps.
Hope everything is going okay?