We're in Month 7 with my father-in-law... stage 4 lung cancer [right lung is closed down], and some lymph activity. We go through waves of what he'll eat [he had radiation that affected his throat; we're past that, and have just finished chemo].
Right now, yogurt and Boost/Ensure are about all he eats. I'm guesstimating about 600 calories a day get into him [half a yogurt here, a couple sips of juice there]. He's also diabetic, so regulating sugar is hard...
I guess my advice is, whatever works, works. If he wants ice cream in the morning, great! Soft-scrambled eggs, lots of caloric creamy soups, protein smoothies with ice cream and fruit. Go for full-fat versions of everything. I usually add protein powder and/or fiber powder, as well.
Good luck! Keep in touch if you need someone to talk to-- we're making our way through this blindly: it helps to have a hand to hold!
~terry