6 month old Great Dane has diarrhia.?

I have a 6 month old Great Dane who is experiencing diarrhia. He went to the vet about a week and a half ago, and we learned he has worms. He is on a 6 month heartworm medication regiment, and our vet provided us with a weeks work of antidiarrhia meds. Unfortunately they didn't work too well. He is still eating, drinking, and acting normal. he just has the runs all night long! I just want to see if anyone else has experienced this, and if I should take him back to the vet. Also, we're feeding him science diet, could that be the cause?