Hi Sarahp and congrats on the new pup!
Couple of things could have gone wrong.
*A HUGE change for him leaving his litter-mates and mom can cause stress upset.
*Changing foods for dogs/pups can take from a few days to several weeks. ALL dogs' are different. You need to start out by feeding maybe a Tablespoon of new kibble mixed with old kibble for one meal and then watch stool. If no loose stool after one or two days, increase the amount of new kibble slowly watching every bowel movement to make sure the new food is agreeing with him, and so on.
*Some dogs do not do well on BB.
Pups should not be fasted for 24 hours....only 12 at the most if necessary.
Try a mixture of Boneless, Skinless, Boiled Chicken Breast cooled and shredded with equal amount of canned pumpkin (NO spices). Some people use rice instead of the pumpkin for this or you could do a combo. Feed several small meals during the day, making sure that the diarrhea is subsiding after the first or second feeding. He may need to stay on this for several days or a week.
Once his stool is normal, you can SLOWLY try to introduce the BB again, BUT if you see loose stool again, he is not tolerating the BB so you'll have to go to plan B dog food. You may want to look at Nature's Variety (Not Nature's RECIPE) sold at Petsmart or Fromm's, sold at doggie boutiques.
Nature's Variety Locations: Find A Store | Nature's Variety
Fromm's Locations: Fromm USA Retailers
IMHO - use a non-grain food.
I've been feeding raw for 20 years but realized long ago that most people cannot feed raw. So my suggestion for now (until you decide about a raw diet) would be to use a high quality kibble, add digestive enzymes/pro-biotics and a good quality "named" fish oil. But for now, concentrate on the situation at hand.