Take a deep breath. I know you are frightened for your fish, and that is ok -- it means you care about him -- but you need to not let that take over. Take a few deep breaths.
Not eating for a day is not the end of the world. Bettas can live for quite a few days without food. Now, it does indicate that something is wrong with him, and you did well to notice that promptly, just don't panic about it.
The heater is a great idea, even if it only warms his tank a few degrees, that will certainly be helpful. Clean, warm water can have amazing effects. If you are keeping him in a cup in the tank remember to change the water every day, and if you do choose to add aquarium salt or epsom salt, remember it needs to be fully dissolved.
If you are mixing his water in a jug (I used to use a 1-gallon jug of spring water from the store, but with tap water, and add the salt then shake it up really hard for several minutes, and even then dissolving aquarium salt was tricky). Aquarium salt is sold at the pet store and you may need to ask for it. Salt used for marine/saltwater fish is completely different. If you decide to buy epsom salts, make sure you don't get the kind with perfume added. Lots of epsom salts include weird stuff like lavender. Go for the plain kind, and read the ingredients list. It should have nothing but magnesium sulfate.
Make sure to remove uneaten food from his water, especially the cup so that the water does not get spoiled.