Lonliness maybe?? He needs something to occupy his time and mind beside thoughts of what happened.

I have been living off of my PEG for >2 years and hear are a few things I have learned: Cancer feeds on calories/sugars, chemo can damage your hearing (a pre treatment test is a good idea), this site is golden for help and advice. Good luck to you and your brother - our hearts are with you both.
I have just started using a living fuel supplement to replace jevity 1.5/isosource 1.5 and it seems good so far. livingfuel.com

Another good site is Lucys realfood. Great recipes for peg tubes and liquid feedings. I have been making my own peg food since October 2009 with healthy food, healthy calories, proteins and fats and very little sugar content. The recipes were approved by the nutritionist and, as a matter of fact, she was very impressed.
I'm gaining my weight back slower than I'd like but I'm told to expect that.

Thanks for the link. I am reading the how to's and have a comment. Instead of rewarming the food in a microwave, it is often easier to bring it to room temperature by placing the container in hot water. The food container should be around two cups and the hot water should be in a container at least 4X bigger. This prevents microwave hot spots.

I will have about 5 days between the surgery and my brother being released from the hospital to go shopping and pre-make some food. I won't know if he is going to have a peg until after the surgery.

When reheating frozen food it should be reheated to a minimum of 140 degrees. You should also never thaw frozen food in still water or on a counter top. Freezing does not kill bacteria, it only goes dormant so it should be either cooked in a frozen state, thawed in a refrig, microwaved or placed under (preferably cold) running water.

Best is thawing in the fridge then cooking as David said.

I'm also 6' and pre-cancer weighed over 200 lbs. Ten years after treatment I weigh 155 and can't get beyond that. Encourage your brother to consume as much as he can prior to treatment. Loss of taste and saliva can cause a lack of interest in nourishment. Then the stomach shrinks and causes a further lack of appetite.
On the positive side, I feel better now, ten years on than I did when packing 215. My grandkids Wii, says that 6'/155#s is normal. So bro is not that far away.
Most important is that you don't nag or put pressure on him. If his weight becomes problematic, his medical team should be able to deal with it. I avoid Boost and Ensure in favor of store brands. Walmart, Safeway and nearly all of the drugstores have house brands that taste better and are way cheaper.

He is taking Boost as a temporary measure. He has gained about 4 pounds which is a lot when someone has been as thin as he is. I am not a fan of Ensure or Boost or any of that type of stuff, but right now it is going to be good enough.

After his surgery when I have him in my clutches, I will make sure he eats/drinks quality.