Hmm.....
Male bettas should NOT be housed together. They will kill each other, or get badly injured. My Bubbles is a perfect example, as he was attacked before and had half his tail bit off (thank God I walked into the room when I did or it would have been worse). That could be a cause of the problem, the other betta might have injured him.
I don't know much more, but I suggest you look at the following sites and see if you can relate what your fish has to any of these illnesses:
http://www.bettatalk.com/betta_diseases.htm
http://lightning.prohosting.com/~cyc...e%20betta.html

Britt