Yippee!! This is great news!
The baits usually contain inert ingredients such as peanut butter, breadcrumbs, sugar and vegetable or animal, which could be attracting to pets. The avermectin is not toxic to dogs and is not going to cause him problems.
Usually, if we see any kind of problems at all, it is from eating the inert ingredients (like the peanut butter) which can lead to mild diarrhea or gastroenteritis.
The container, if swallowed, could cause a foreign body obstruction if it got wedged into the intestines, but this does not sound like an issue with your boy as he did not swallow it whole, but chewed it instead.
So, I would with-hold food for tonight, but offer water or other clear fluids (you can dilute 50:50 with water any of these: apple juice, gatorade, pedialyte or chicken/beef broth). Tomorrow, keep an eye out for soft stool, but I do not anticipate any problems.
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Best wishes! Fiona