Hi Owric, I think the vote is in: your guy probably doesn't have ich but he definitely is in need of a heater and he's pretty bloated. You just can't trust the instructions on the fish food containers 'cause your betta can eat a LOT in five minutes. Bettas are greedy guts. For a betta, one very small flake is more than enough for a meal. You might want to consider switching to pellets because it's a lot easier to judge how much you're feeding: one pellet or two. Also consider adding in some frozen or freeze-dried bloodworms or brine shrimp for a meal or treat. Variety and small, frequent meals are best when feeding bettas.
Try Arashi's method for getting him to pass some waste. I've never tried it but it sounds like a pretty good tip since I have noticed my bettas leave behind . . . er, souvenirs after I chase them trying to get them into a cup for water changes etc. If perchance this doesn't work, then post back and we'll discuss further treatment. Right now the most important thing is to get the bloat down because it can lead to all sorts of bad things that may not be reversible.
Once you've gotten a heater and you keep the tank at a steady 78-80 degrees farenheit, it will be important to make sure the water you are refilling your tank with during water changes is the same temp as the water you took out, give or a take a few degrees. Bettas are from tropical waters and need warm water to thrive. In colder water like yours is now, he's using all his energy just to stay alive so he doesn't have any energy left to swim around. That's why he's so lethargic. Warm water will perk him up.
I would suggest keeping an eye on those white spots on his head. They could be any number of things, most of them benign. But they aren't ich so that's good.
I hope these tips work and I hope your little guy gets better quickly.
Also, welcome to the forum. It's good to have you.