Sorry your fish is having problems. How long has it been since he has eaten? Fish can go for many days without food and come out just fine. I would leave the water alone for awhile. Lots of sudden changes can stress a fish out. If he shows no outward sign of sickness it is usually best not to medicate him with anything other than the standard meds you have given him. Bettas are also really picky eaters and will sometimes go on self proclaimed fasts for no particular reason. Has he recently been seperated from tankmates? Some will stop eating when they are seperated from other fish. Give him time and prayer and hopefully he'll come around. :)

Fish often get lethargic and eat less if it's cold and 70 F is kind of low for a betta. Try raising the temperature in the tank to 75 F or so. I had one fish that refused to eat and was not very active until I raised the temp. Now, he's pretty hyper. Good luck!