Every dog is different, but if your vet is recommending prescription food, I would go with that for now.
In my experience, dogs with sensitive stomachs have trouble digesting potato (cheap filler)
That is why I like Zignature, you can try the different formulas and that will be considered a rotation diet, I prefer to stay with one brand when dealing with a dog that has issues.
For a more reasonable option, I have heard good things about Pro Plan Focus Sensitive Skin and Stomach kibble.
I use no supplements except a daily fish oil capsule for my dogs.
Also, I would wait till he is stable (no more loose stools) before putting him through the neuter.
