Looks like you have some work to do.
I suggest getting a 25 watt adjustable heater. A betta is a tropical fish and require a constant temperature of 78-82° to thrive.
You need to be doing more water changes. At least 1-50% and 1-100% water change weekly. I would suggest doing 3-50% a week. I'm not a fan of 100% water changes. Also a switch in water conditioner will help. I recommend using Seachem Prime. Add 2 drops per gallon when you condition your water and 1 drop per gallon a day to keep the ammonia build up in check. Prime can lock-up the harmful ammonia for 48 hours until you can do a water change.
You will need a test kit. I would recommend getting an API Master Test Kit. This way you can accurately keep track of your water parameters
You might want to consider a filter, it will allow the tank to cycle and create a healthy and nearly worry free environment for you and your fish.
You can read about cycling here, http://www.bettafish.com/showthread.php?t=111960
When the tank is cycled you can reduce your water changes to 2-50% a week.
You can use a hang on back filter or a sponge filter. I personally use sponge filters. You can see mine set-up here,
I would suggest switching him to a high quality pellet like Omega One Betta Buffet or New Life Spectrum betta, community or surface feeder formula. The are high in protein and low on fillers. Once you get the temperature up a would suggest daily feeding with a fast once a week. My little plakat gets 6 pellets a day.
He is a beautiful fish. Do the best you can.