Yay for the weight gain! I'm so glad things have been going as well as they have.
I don't know if this information will be useful to you, but since prednisone melted weight off our then-7-year-old, he was switched to budesonide, which is also an immunosuppressant but targets the gut more selectively and has fewer systemic side effects. This treatment is for IBD in our dog, though, so I don't know if it's useful for something like cancer.
It would be interesting to know what the OSU team would make of the ultrasound and whatever other signs the oncologist is using for the diagnosis and prognosis. Are they going to look at those things?
In any case, you, Harrison, and your family are in our thoughts and prayers. I admire your efforts and devotion.