Try ensure mixed with ice cream in a blender. I don't like the taste of ensure either, but mixed with some ice cream makes it taste better. Might try adding some fruit in a blender with it.
My gastro doctor said eat anything and everything I can to keep the weight on. She is working with my oncologist. She also has me on hydration every day with potassium and magnesium in the hydration (I have sores in my mouth and I cannot swallow my pills right now). This has helped with issues concerning my blood work too. When we get dehydrated, it messes with other things in your body. Home healthcare brings me all my stuff. They also come to the house and do my bloodwork so I don't have to travel. I can drive to the doctors, but she wants them to come to my house.
Another thing for the yeast in the digestive tract - Thrush - mouth sores, is ask for the miracle mouthwash...it can be made up different ways. My gastro doctor gave me the one made with lidocaine, nystatin, and benadryl. My oncologist gave me one with lidocaine, maalox, and benadry, and a separate bottle of nystatin (this is the one for yeast, thrush and mouthsores-it works). I take the one with lidocaine, nystatin and benadry right now. Easier to use one bottle instead of two.
Cancer cells do like sugar and dairy. But he needs to build up before worrying about that right now. Fruits, veggies, meat, gravies, milk, weak tea decaffienated, coffee decaffienated, ginger ale, coke if his stomach is upset and he can tolerate it, and I drink lemon-lime (yellow) gatorade with lots of ice. The gatorade will help with replacing some electrolytes, as well as an adult version of Pedialyte...I just don't know what that would be. Get him built back up first. then worry about the no-no foods.
After surgery, we all lose weight for a few weeks unil we stabilize. I think it is such a shock to our system.