Edit: Your fish really needs a heater. Badly. It's been repeated by several members who have been trying to help you. Is there a reason why you can't or won't get the heater despite the numerous advice given on this?
Edit again: If finding a heater to fit in your tank is a problem, or you can't find small enough heaters that will stay constant, you could always try using a desk lamp above the tank to warm the water. It's not a recommended method especially since it's difficult to control the temperature and there have been cases where leaving a light on for too long might overheat the water, but your fish isn't going to survive very long at the cold temperature you've been keeping him in either.