Switching around food too many times can cause ulcer and other issues. So once u find one brand which works stick to it. Switching between brands always requires careful transition. Same brand, different protein is normally ok, but with a puppy and stomach issue history, better to be overly careful.
Get as soon as you can raw goat milk (we use primal) and give him that. If you only find the powder version, that should be fine too. Wait 4-6 hours, cook some sweet potato, remove the skin, wait until it is lukewarm and give him that (i put that in a kong, my boys love it, that is one of their favorite treats). For the next meal, get some ground turkey, cook it, give him that. Once u see the issue clearing up, start adding kibble (gradually, start with just 10% of what his normal portion will be, gradually removing the turkey). Turkey is also a helpful item for keeping him low while the healing process is ongoing.
It will take some time, so be patient, even if it means for several more days he won`t eat his full amount of food.
Suggest not giving him any commercial treats. There are some which are 100% turkey or salmon, freeze dried, in my experience even sensitive stomach can deal with those. I give mines every evening 2 pieces of baby carrot too, that is there post dinner snack, they love it. They also get a piece of cooked salmon once a week, again in a kong. I add pro and probiotics for every kibble meal they get.
Hopefully this helps, i know it is devastating to see your baby suffering. Let me know if any other questions please.