Ok first of all... NEVER FLUSH A LIVE OR EVEN SEMI-LIVING FISH DOWN THE TOILET!!! I don't mean to yell at you, but this is something I am very opinionated about. That is the worst thing you could do to try to put a fish out of it's misery. If you need to kill him humanely, hit me up on icq and I'll tell you how I do it. (78989718)
Secondly, from the darting around, it sounds like it may be internal parasites. When you do his next water change, add some aquarium salt (if you don't already normally do that) along with a couple of drops of liquid parasite medicine, and be sure not to overdose him cuz that could be fatal in itself. You might also want to treat him for a secondary bacterial infection, but first I would concentrate on the parasites cuz that will kill him.
I've had fish all my life. I have 2 bettas currently, and here's a site that I refer to from time to time when one of them seems to be not feeling well:
BETTA TALK
It's got some pretty useful information on it. The main thing though is that with any sickness or disease, you have to catch it early on or it's almost certainly a lost cause.