Also, and I hate to say this, but look at the ingredients for your fish food. The tropical fish food might be veggie based fish food for general tropical aquariums. Bettas are carnivores and need meat based food. Look for ingredients that include fish meal.
Pellets specifically for Bettas are the best thing I've found for my fish so far. We also feed the Aqueon ones. They're small, and you can get them at Petsmart.
If he's too cold, it's probably killing his appetite. For a small tank, I had nice luck with the hydron mini heater. It's a tiny black pad that goes under the pebbles and it kept my small aquarium nice and toasty, and it was really cheap. Do get a thermometer though. It helps to know exactly what temperature you're dealing with, and you can go from there.