I have 6 bettas - 2 females and 4 males. None of them will eat the flakes they all prefer the pellets. Also it is possible for a betta to go up to 30 days without eating - not recommended but they can. You can get small heaters for bowls or tanks up to 5 gallons - they are under the rock heaters. Our Petsmarts carry them - just make sure you have a thermometer to keep a watch on the temp. Some grocery stores and wal-marts carry fish supplies you might check to see if they have a pellet food in stock for bettas. Wal-mart also carries small heaters. A betta will do okay at water temps around 72-74 degrees, but, will thrive in temps around 76 to 78 degrees. Make sure that you are changing the water once a week since he is in a bowl - dirty water can cause not only illness but depression. Getting him a "friend" like a mystery snail might help as well. Remember that you have to feed the snail if you are cleaning you bowl out - there will not be enough algae for the snail otherwise. With his bowl being near the window try to make sure he is not in direct sunlight and that his water temp is not changing to much - this can stress them out. Try and keep the mirror time to around 10 minutes at any time so that it doesn't stress him out too much. Same with a new male betta - some males stress if they can see each other (they can never "win" their own space since the "loser" can never leave), other males seem better if they can see each other - so see how they do with each other. Otherwise you can let them spend time together each day or so for short periods - just make sure that you are never moving one male away each time he can get depressed if he is always the loser.
I know someone in an earlier post suggested betta forums for information - they are great, even if you never post just reading will help. Good luck with Sushi.