Hi Brenda E-
Well, sounds like a stressful situation. Have you tried feeding small meals of boiled chicken or burger with rice and pumpkin to rest and calm down his intestines? Then slowly start adding kibble back in. My pups had diarrhea when we got them, but they did have parasites. Both Giardia and Coccidia. Hopefully your vet sent a fecal to the lab to test specifically for these. Yes, a probiotic would be good to try. I use Vetri-pro BD by Vetri-Science. I order it from either Amazon.com or healthy pets.com. It contains probiotics, digestive enzymes and some supplements that help soothe the intestines.
Your large breed pup needs a puppy or all life stages food that is appropriate for his size. It is recommended to keep the calcium levels down for the growing stage for large breeds. There is more information and a spreadsheet of recommended foods created by Hound Dog Mom on the large breed thread of this forum.
You will need to keep his growth slow and steady to avoid joint issues. Also limited exercise is important.
Check out http://www.dogaware.com for a lot of information on digestive orders. Good luck!
