With loose stool problems like you’re describing, I will give a dog a something like cottage cheese and white rice, bland & highly digestible, for a few days. (You could also use boiled plain chicken breast for the protein.)
That usually will clear up whatever is going on. And it allows the dog’s system to recover. In fact, often you will not see the dog produced stool for a day or so, and then it will be minimum. Make sure the dog has sufficient water, whenever a dog has diarrhea (or vomiting).
I really doubt that your dog needs the specific Iams food, but that mix of foods from 2 Wellness products and Orijen (why the mix?) may not be right for your dog. Try a single recipe blander, simpler, moderate in protein & low to moderate fat diet for a bit. You need low residue, which is just another fancy way of saying highly digestible, but it doesn’t have to be a specific formula & brand with that in the title or be a vet diet. Watch the treats too.
Do not overfeed, underfeed until you get it right, divided between two meals a day (no free feeding), and watch to make sure your puppy does not eat too fast. Do not play/exercise/walk/have excitement for your dog within 1-2 hours of feeding, backwards & forwards. Feeding should be during a time of calm & rest.
A tablespoon of all natural plain yogurt, with live active cultures, can also help with problem stool/digestive upset.
It’s good that your vet gave probiotics, as the antibiotics destroy beneficial bacteria. I don’t typically jump right away to antibiotics and would encourage you not to do that either. They are overused, cause problems, and rack up your vet bills unnecessarily.
