Cancer treatments are more akin to torture than anything else. All the reasonable discussion in the world will not convince the body and the Id that it wasn't being attacked. These therapies can also destroy (hopefully only temporarily) the taste buds and the body's ability to discern hunger.
He must get hydration. It is crucial. Offer him water or juice first thing in the morning. Keep a glass by his bedstand. Try applesauce or jello or ice chips.
Try juicing to see if your hubby can't keep some food (and liquid) down that way. He is tired and for a little while it would be helpful to relieve his body of the work to digest food. Try a clean cucumber, 3 celery ribs, and cup or so of pea or sunflower sprouts as a juice. The mix is bland enough that he ought to be able to keep it down and not complain. At the same time it is protein and energy rich. If he wants some flavor, add a sprig of mint or a little bit of ginger or a squeeze of lime. This ought to strengthen him some.
Tell your idiot oncologist that your hubby needs some therapy, that he is not bouncing back from the treatments and you want a physical therapist to work with him. Sometimes hospitals have direct connections to specialized gyms where physiologists are there for folks in traumatized conditions.
It was actually in the midst of my first surgery, then chemotherapy, and then radiation, about 8 weeks in to 16 weeks of radiation, that I realized there was no turning back and that I was in a ruined condition. Once I was satisfactorily burned crispy and had projectile vomited through another round of drugs to my bloodstream, it took me months to recover enough to even consider how to regain any physical ground.