Exactly what pellets are you feeding? For now, don't feed for the next 2-3 days and watch his poop. Report back on what it looks like. I would also use epsom salts - 100% pure magnesium sulfate with no dyes perfumes or additives of any kind. Predissolve at a rate of 1/4-1/2 tsp per gallon and then add slowly to the bowl. You only need to replace this with a water change and then only add back as much salt as water you replace.
Heat lamps are not appropriate to heat fish because it causes swings in the water and those swings are hard on fish, and can even be lethal.
2 gallons is minimum for keeping bettas long term. Bettas kept in 1 gallon bowls will live an average of 2 years, as opposed to double+ that in larger well kept containers. This is because ammonia builds up too fast in the 1 gallon bowls and you could even see as much as .5ppm of ammonia in as little as 48 hours.
A 2 gallon can accommodate a good adjustable 25w heater. I suggest a Marineland Visitherm or Jager. I know Petco has Topfin that people have used successfully. You need an in tank thermoemter: http://www.amazon.com/Marina-Floatin...na+thermometer
If you keep the one gallon bowl however you can't use a good adjustable heater. You'll have to use one of those tiny preset ones.
You should always give any new heater a 24 hour test run in similar size container with thermometer to monitor temp and make sure it is holding one constant appropriate temp. Once you have determined that the heater is working properly you can slowly warm up your fish. How you do this depends on if you got a good adjustable heater or one of the preset ones. But you cannot just place the heater in the tank and let it heat up fully with your fish in there.
Bettas are topical fish and need to be kept one stable temp of 76-82, but 78-80 is ideal. They need to be placed on an interior wall away from exterior doors, windows and any air vents as these will all cause swings.
For a 1 gallon (or even 2 gallon) you need twice weekly
water changes of 50% and 100%. Using Prime or AmQuel+ as your water conditioner will help reduce the toxicity of ammonia they will see between changes but it will not completely remove it nor will it replace a water change.
To do this without shocking..
*Use a turkey baster (new rinsed no chems) to remove 50% of his old water and as much of the debris in the gravel as you can.
*Use tank thermometer under the tap to match tank temp) and fill up gallon water jugs (like you get at store.. rinsed with hot water but no chems), add your conditioner, shake it up a bit to distribute the conditioner, and then use this water to refill the container.
*If you're doing the 100% he will need to be removed by using an opaque plastic solo-type cup and sat aside somewhere safe. I also suggest wrapping it in a towel for warmth.
*Use same temp water like before but this time you will fully empty the bowl and rinse everything off including the gravel - no soap or chems.
*this time float in the cup for an hour, adding a couple tablespoons of the new water to his cup every 10min before releasing.
At this point you need to get on top of your water changes
You need to get him warmed up
You should look into getting a 2 gallon+ for long term health
You should fast for now, use epsom salts, and watch poop