Puppy diarrhea

I have a 6 month old bernedoodle who has been having diarrhea and loose stools. He came home from the breeder on TCL puppy food but we transitioned him to royal canin large breed. He was fine until he went to the boarders (it was actually his puppy class instructors house) for the weekend while we were away. There were several other dogs there while he was there. 2 days later he had explosive diarrhea. He went through 10 days of flagyl (fecal exam negative) and a bland canned diet and was better but stopped eating his royal canin. 2 days after the flagyl was finished he started the diarrhea again. Took him back to the vet and fecal exam still negative. But now back on flagyl plus Imodium and chicken and rice diet and I added a probiotic. After 5 days of chicken and rice and 3 days of formed stool I started transitioning to taste of the wild salmon puppy food (he doesn't like royal canin anymore) and after 2 days he had a soft stool again. He acts fine and playful and loves his food. We have given him only a small amount of totw (less than 25%). Any suggestions to help my pups stools?