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Hello. Thanks for the question.
The diarrhea may be secondary to the heartgard if he doesn't tolerate the beef flavor tab. If that's the case, you may want to switch and see if Sentinel is better tolerated in the future.
You don't really need to run him to the vet, as long as he's still eating/ drinking. You can add some fiber to his diet to firm up the stools and get the colon moving more normally. You can give 1 tablespoon of canned pumpkin 2-3 times daily as a fiber source.
I hope this helps, let me know if you have any other questions.