I'd like to expand for a moment on what Marie said about shakes as meal replacements. I had a conversation with my RO on this subject, and he cheerfully advised me to stay on them as long as I liked as long as I made sure to get enough every day. Healing is a high-energy activity.
Anyway, I was on them for more than 90 days. I was fortunate in that I totally lost my sense of taste very early in the process, so I was able to drink Jevity (the canned formula used to feed patients with a PEG (feeding tube)), and my RO just happened to have a couple of cases left over from a Patient who never wanted to see the stuff again.
(I was less fortunate in keeping it down, due to the extreme nausea from Cisplatin, but that is not a problem related to or caused by the liquid, as opposed to solid, nutrition.)
Best of luck to your dad, he's lucky to have you in his corner!