Was his top fin always twisted? Sometimes they get that way from hard water, especially if it's a CT. Though, def. do a water change and check your nitrates (and ammonia, if applicable), since that's not normal.
Temp should be 75-82'F in it's tank, too, and will succumb to cold if not or if poorly regulated. If it's in a community tank, put it in a tupperware cup and float it in the display so you can keep an eye on the betta and also keep anything nasty from hitting the other fish. Just watch the params in the cup but since it's a betta, it doesn't need aeration, which is nice.