Hi 2doodlemom-
If you are having trouble getting your dog to eat and she is TRULY underweight, I would ask your vet if he is comfortable prescribing Mirtazapine.
It is used as an appetite stimulant in dogs and cats and may be helpful in jump starting your dog to want to eat again.
One food that I find typically settles well with dogs with sensitive stomachs is Purina Pro Plan. This is a food that may be worth trying for your dog. I find it is palatable and easy on the digestive tract and they make a sensitive skin and stomach formula that I used for a long time for my dog with GI issues.
Since your dog is almost 2 and having trouble gaining weight (again if he really is underweight and not just in good physical condition) blood work is always a good idea. If anything this gives your vet a baseline and if something comes up later they can look back and see what normal looks like for your dog.
