i think the warm water fluctuations might be a little hard on him, harder on him than keeping it steadily cooler than he'd like. they can deal with cool water for a time, just not forever, so i'd avoid the fluctuations. i'm a fan of the shatter proof marineland stealth heaters. they have 25 and 50 watt ones (they're about the same size) that would be good if you decide to upgrade tanks. (I think the 25 watt is good up to 8 gallons; i'm not sure on the 50). I've successfully used the 50 watt in a 1 gallon hospital tank. it's a little more expensive as far as heaters go, maybe $28 to $35 if you can find a sale. There are also glass heaters that are half the price, but I don't have experience with them. I prefer a heater with a temp dial on it. other ones will just nebulously tell you they'll raise the temp N degrees above room temp, but i'm not comfortable with that.
i've heard lots of bettas dislike flakes. i like hikari betta bio gold pellets. you can also treat him with freeze dried bloodworms once or twice a week. if you check under the betta care section a lot of people have discussed food choices. hope that helps! good luck! post pics of the little guy and his home if you can!