Your brother must not have cleaned his room very often or at all. You will have to clean everything--that means baseboards, windows (inside and out), curtains if you have them, carpet, and you will probably have to re-paint the walls. Start by removing the furniture and cleaning the window frames and baseboards with a commercial cleaning product (go to Home Depot or Lowe's and get a heavy-duty cleaner). Then move to cleaning the windows. Take down the blinds or curtains and either get them cleaned or replace them. (Get some cheap ones at Big Lots). Go over the walls with Kilz and a paintbrush. Kilz kills mold and mildew. Then paint the walls. (Remove the baseboards when you're doing that or cover them up). If they still smell after you've tried cleaning them, replace them and paint them as well. Finally, rent a commercial carpet cleaning machine and clean the carpet. If the dirt and smell is too ingrained, rip out the carpet and replace it with stick-on carpet squares. I'm sure the pad under the carpet stinks as well. Remove it and replace it. Finally, move furniture back in and give it a through cleaning as well. If it's wood, use lemon oil. If it's plastic, wash it with soap and water. All of that should give you a fresh-smelling place to live.