Notself

While it's all good advice, my suggestion to you is QUIT WORRYING ABOUT WEIGHT. Yes, take VHC or Ensure along with water, but my personal experience is that your brother can thrive (let alone survive) even if his BMI is "underweight". I am 5 foot 11 inches tall and weigh 134 pounds. Yet I can bench press 140 pounds and generally out work "normal" men of my age with zero difficulty. I shoveled snow for 4 hours at a stretch while my neighbors were exhausted much earlier even though they were younger and "healthy"
The key is lean muscle mass and instead of taking chances on unregulated and potentially contaminated and dangerous "body building" protein powders etc, just do a Pilates or Yoga class regularly AFTER TX.
My wife worried herself sick when I went from 177 pounds to 134 and the doctors did not help any, instead they compounded the problem. IMO our obese society's pervasive denial of how fat and flabby the majority of people are (including my Cancer center doctors) generates this false worry.
Life extension research comes down heavily in favor of substantial reduced calories (although it has only been proven in mice). The best thing you can do for your brother right now is to make sure he has a pre TX TSH test so he will know what his thryoid levels should be after TX. Now that is a real problem.
Charm