I would absolutely just feed one thing until you are sure that’s OK and then just add one new thing at a time. That’s what you should do for an allergic dog, it’s called an Elimination Diet. It’s the only way to be sure what he’s allergic to. Do NOT discount the idea that the vegetables can be the issue. I just discovered that my dog is HIGHLY allergic to peas and I’m pretty sure green beans too! I had tried to give her some garden fresh green beans and she had a reaction the next day. They are both Legumes.
I would not give any vitamins or supplements etc during your Elimination Diet either because they can also be allergic to those!
