hi HC
maybe it is the most important thing now to know what kind of cancer is it, yin or yang ? which area of the stomach is it affecting, the upper part (fundus) or the lower part (pylorus)?
the first affecting the more yin ,expanded region, where the hydrochloric acid is released and is primarily caused by long-term overconsumption of yin foods (white flour, sugar, honey...etc)
the second is occures in the more yang ,contracted region ,where the pepsin is released and primarily caused by long-term overconsumption of yang foods (meats, eggs, pork...etc)
since each case has its almost opposite solution, they both agree that the patient must avoid both extremes.
if your father cant absorb any food ,and grains are the most important ingredient in the diet, what do think about BROWN RICE PORRIDGE ? it may be helpfull in your father case
i dont know much but i wanted to help
hope luck for your father
please tell us about him regularly

note: almost all what i said was experted from THE MACROBIOTIC PATH TO TOTAL HEALTH by MICHIO KUSHI and ALEX JACK