The best thing to do is to buy him a heater, that's really the only thing to get the water how it should be, which should be around 78-80. To buy immediately you can get like a marineland stealth heater at like petco and petsmart, but they can run pretty pricey and even the cheapest heaters at pet stores arn't any less then like 18 bucks. Don't get preset! The cheapest and nicest heater I know of, and the one's I use and so far work well, are Hydor heaters and you can buy them on Amazon.
If you click the "two new" button, you can get one for 11.49 and like six dollar shipping. I got mine in three days after ordering, so the shop Amazon uses to get it ships very quickly. Make sure you buy a thermometer as well to keep an eye on the water temp when you have the heater. To keep him warm till then, well to maybe at least cut the chill from the water, I would suggest finding the warmest room in the house, bundling the tank in a blanket (put it on a blanket then wrap it around so it's covered all around) and make sure if you have the heat running in your house put him close to the vent, but not directly under it. If you still have the air running, then I would suggest closing the vent. If you have a space heater, I would use that too. 70 is very cold for a Betta : ( But that is only a very temporary fix, and it probably won't raise the water temp more then like a degree or two if that, but it's better then nothing, but you really do need a heater.