Hi
My dad who also had colon cancer had the same problem, and what we found was that if you give him his favourites, he would tend to eat more of those, though the intake was still far less than when he was healthy. Still, it's better than giving him foods he doesn't like - when he will eat nothing at all.
However, your dad may be developing what's called cachexia, since he's already on his 18th chemo treatment, and if this is so, there is no point force feeding, as you are actually nourishing the cancer cells rather than the normal cells.
Try to give him pure fruit juices, juiced from the actual fruit, and not the store bought type and lots of liquids throughout the day. It's easier for them to take in and digest liquids as opposed to solids. If he likes the fruit juices, give him as many glasses a day as he will tolerate.
The sad fact is that they will eat less and less, and not feel hungry. It's very difficult for the family, who feel that they must force the patient to eat, but it's actually of no benefit, even if they do, for there comes a time, when the body cannot absorb or glean any nutrition from the food, and your are actually nourishing the cancer cells.
The best thing to do is keep him comfortable, and we found that hospice home vists are very helpful both for the patient and caregivers.
Audrey