All dogs that are sick can be enticed to eat a meal or two, something new, especially if you cook for them.
From the information provided (read the OP’s original post) this is not a senior dog, poor appetite is unusual in a dog this young…unless something is wrong. Plus the dog is underweight.
So, instead of trying this or that, I would have whatever diagnostic tests that your vet advises, asap.
Regarding the need for an endoscopy, I have never had one done on any dog I have owned, that would be an extreme measure. An invasive procedure.
Usually a basic GI workup and labs will lead to an accurate diagnosis, then you can go from there.
Best of luck
Ps: A biopsy is usually done with a very thin needle, takes a second. Just wanted to correct some incorrect info I saw (I ignore as much as I can) plus your vet may not think it is indicated.
