Actually it would be easier to do 100% water changes if you do it this way.. Get a small cup to scoop him into. Scoop him out, dump the water out, put in a little water to rinse the gravel around until its clean and dump the water. Regulate the water temperature to the same temp as his tank using your thermometer, fill the tank back up, add water conditioner and put him back in. It should only take about 5 min for a tank that size. :). It's also important when doing water changes, that the temperature of the new water is the same temperature as the old water you dumped out. Otherwise it can stress them out.
If you can, I'd look into getting him a little bit bigger tank and heater when it starts to get cool out. You can't really put a heater in a 1.5 gallon tank and bettas do best at a temperature of 78 – 80°. Hope this helps.
One more thing- make sure your cover your tank because bettas like to jump!