Did you have your fish with you during the break? Or was it in the dorm? Your lucky your fish is still alive, but he is suffering a lot. This very cold temp has weakened his immune system and at this point he's just struggling to stay alive. Wrap his tank in blankets, float a warm water bottle and go get him an appropriate size tank and heater.

This chart shows the heater size needed for the temperature increase you need. According to the chart you need to get a 100w heater and a 5 gallon tank. A 50 w heater increases water temp a max of 15 degrees, and you need more than that, so you need 100w. If you're in the US check prices online for petco and petsmart. Online prices are often cheaper than in store prices and they will match them.