Sir Butters is having tummy issues! Help!

His breeder recommended grain free food which I know now is bad. I went to Petsmart to replace it and asked the worker his recommendation for grain free for a golden puppy. Came home and literally see GRAIN FREE on the bag… my fault I should have checked. He has awful gas and his poop isn’t quite diarrhea but it’s not solid either. What foods do you recommend? Preferably something I can buy at Petsmart.