Try giving him plenty of space to move. A 1 gallon is the minimum but 2.5 gallons is much better

Try reading about bettas on these sites:

http://www.ultimatebettas.com/
http://www.freewebs.com/betta_fish/index.htm
http://www.petfish.net/forum/index.php/board,8.0.html
http://www.petfish.net/kb/9/

Hope that helps