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.