He really needs to discuss this with his doctor. A lot of times, primary care doctors that do not specialize in this type of disorder does not know enough about it to give the best treatment for it. If he isn't seeing a gastroenterologist, then I suggest he do so and ask this doctor all his questions. Write them down ahead of time so he doesn't forget anything.
There is a fatty liver disease that does progress called NASH. Since he doesn't have any of the other problems associated with fatty liver like alcohol abuse, obesity, etc., then he might like to do some research on this and ask his gastroenterologist about it. NASH is non alcoholic steateopathic (spelling?) hepatitis. This disease happens when a fatty liver keeps progressing until it eventually will hurt the liver. It can progress all the way to cirrhosis in some people. Did his CT scan show an enlarged liver, enlarged spleen, shriveled up shrunken liver, etc? Only his doctor can answer this from the testing he received.
I had cirrhosis for no known reason myself and had to get a transplant. I think I was diagnosed with just about everything to be the cause at one time or another. I was told it was possibly autoimmune, NASH, primary biliary cirrhosis and finally they just settled on cryptogenic which means unknown reason. I was told by my transplant people that quite a number of people have cirrhosis and just do not know why. So I went for years not knowing why and then received a transplant. I guess they did quite a lot of testing on my old liver and finally came to the conclusion that it was definitely due to PBC (primary biliary cirrhosis). This happens because my autoimmune system attacks the bile ducts in my liver which led to cirrhosis and liver failure.
I think your brother needs to see a specialist and go from there. I would think that severe does mean his liver is experiencing a lot of active inflammation. Over time, inflammation can progress to fibrosis and then to cirrhosis. That's why it's so important that he address this problem now and try to stop any further progression. Since he doesn't have any of the typical reasons for having a fatty liver, they are going to have to do further testing to figure it out and treat it. If they haven't given him a biopsy yet, then that should really be his next step. Sorry I can't help you more.
liver transplant recipient and nurse