You know, we went through someting similar with my then 10yo DD a few years ago. It started off like any other virus, but she couldn't seem to shake it. She tested neg for all the usual stuff, but her fatigue just grew and grew. It got to the point that she was only out of bed for a few hours a day--if she hadn't been homeschooled, she would have failed! Her joints hurt, her throat, back of her neck. We had her seeing doctors once a week at some points. After extensive testing with the ped, the allergist, & the rheumatologist the conclusion was--it's a virus. And that's exactly what it was. It took her 9 months to recover from the virus and she never did test positive for anything that could be treated. Now DD is almost 13 and only occasionally will she have residual joint pain, almost like arthritis, but it doesn't keep her still.
I hope they can find out what is wrong with your brother. It's sad to see a young person laid low by illness for such a long time. I'm sure your parents are worried and are having him tested for everything serious. Best of luck to you all.