I think the first thing you could do to improve the quality of his life is more water changes. .7 gallons is really a half gallon. I would be doing daily water changes at 50% a day in a tank that small. But if you could do 50% every other day, it's a good start.
The room may seem warm to you but may not be appropriate for a tropical fish like a betta. I like to keep mine at 78-82°. Do you have a thermometer? Without one there no real way of telling how warm you tank is. Heating a tank that small is a bit of a challenge. If we could get him in a 2.5 gallon tank we could use an 25 watt adjustable heater in the tank. There are very few reliable options for a .7 gallon tank as far as a heater goes.
As for a water conditioner, I'd like to recommend Seachem Prime. Since you don't have a filter 2 drops of Prime per gallon daily will detoxify ammonia until we can get him into a bigger tank and get a filter running.