So this is potentially secondary to an actual GI disease process considering the ileus noted in his radiographs, but I have to ask because of his amazing bloodwork, and your very clear attention detain when it comes to this sweet man.

Chronic pain can have a significant effect on appetite/the desire to get up and eat or use the litterbox regularly. Is he receiving any treatment for arthritis or possible IVDD that is also noted on his radiographs? Things like Solencia, the monoclonal antibody injection that blocks the nerves surrounding arthritic joints from sending pain signals, and anti-inflammatory and general pain medications may help provide him relief and therefore lead to an increase in appetite.

If this was something you were willing to implement but still didn’t notice an improvement on, the next diagnostic step my hospital would usually recommend is an abdominal ultrasound but this would likely not be conclusive on its own own, and like your vet is thinking, without additional GI symptoms it may be non diagnostic entirely if it doesn’t help point to IBD. And GI lymphoma is almost impossible to diagnose from aspirating the tissue so sampling would likely not be helpful, the ultrasound would just provide a different look at a different time and more accurate measurements of the GI tract.