How do I go fast down wind?

I sail a topper that's second hand and the previous owner raced in the worlds several times, but when I race I do quite well mainly leading around the windward mark but I get overtaken down wind. I sit forward, lift center board out and let my sail out, like everybody else. But they catch up, I get overtaken by a 14 yr old in an old rubbish boat. Help! What am I doing wrong! The 14yr old always wins, with me just coming a close second / third. He's an arrogant sh*t (but my best friend) and I NEED to beat him. Please Help!!!! By the way I'm twelve