You may not like hearing this, but the best thing you can do for him is be a supportive friend and try to make him happy when you get the time to. You are not an expert on brain cancer. You will not become an expert on brain cancer before he dies. You know who is an expert on brain cancer? His oncologist. I know grief is hard and you're motivated by your love for the people in your life, but there isn't some cheap hack that will fix this. Trust the experts to do their job and for the love of god don't take cancer advice from strangers on the internet. In the meantime, love your family, love your friends, love your life, and try to make others happy.