Resection of the whole stomach means reduction of the food sack where your father's ingested food would otherwise be stored for further digestion. So vomitting does happen if he's given food in normal or even lesser quntities. He really needs to start with very small portions of the food at a time as there isn't enough space in the GI tract. Corn
meal would not affect his diabetes
as the amount he's able to digest is so less that in fact you need to make sure he doesn't go into hypoglycemia
because of the less carbs and/or calorie intake. Try soft and palatable meals made out of corn and do supplement him with vit D, Vit B complex and Vit C in majority- all these available in most of the countries as OTC medicines and you should prefer buying only syrups (do not worry of the negligible sugar in the syrup) as digesting the tablets would cause his trouble again. The other vitamins
would be advised to you, if required, as and when his surgeon/ oncologist
sees him for further followup. Hope this helps you. Let me know if you need to know anything else about this.