While I agree with everything already said about nutrient dense natural foods being the way to go when possible, it sounds from the OP like maybe your FIL isn't at a good point for doing that right now. He is in severe pain, and cannot eat even as much as your two year old. Has his doctor spoken to him about a feeding tube? During my DH's bout with throat cancer he had one inserted. Truth be told, he did very badly on it, and had it yanked early, and just forced himself to eat. Other friends, going through similar issues, have been able to tolerate it much better. It requires using a formula, and not Real Food, but maybe that is what your FIL needs for awhile to bring him back up to a point where he can tolerate food by mouth better. You might also see if your local cancer center has any booklets on encouraging patients to eat. I got one that was quite informative (sorry, can't recall the name at the moment); I altered some things to fit better with what I know of healthy eating, but many of the suggestions were helpful.