Sorry to hear your clown is behaving similarly. It's really disheartening. I haven't found anything conclusive but there are some articles/forum posts with similar symptoms. Here are a few of them, they kinda all give different causes:
Exposure to certain PVC
plastics that leech chemicals, such as garden hoses: Link
Genetic anomalies - "if other fish/es not malaffected it's likely not the environment but this specimen per se... Perhaps a developmental disorder. How to put this... fishes aren't as "done" in terms of neurological progress as mammals when they appear "older"... your one fish may have a genetic anomaly that is now just expressing itself" (WetWebMedia, Bob Fenner
Some kind of virus, possibly lymphocystis: Rare Clownfish Forums
"whirling disease" - Yahoo Answers poster's experience
In my situation I am leaning towards some sort of chemical thing, because I do remember using a new pair of gloves in the tank a month ago. The symptoms seem to fit but I don't know why none of the other fish are affected. With your maroon clown, I'm betting it has a lot to do with his fins being chewed. I don't know much about diseases and treatments, so you might get more help if you create a new thread. Maintain good water quality so he can heal. I hope your clown gets better!