Some people like Hydor. I have never had a good one. I use 25w Marineland Visitherm in the smallest of my tanks. Jager is also good.
I wouldn't use an airstone in something so small unless you have an airline control valve that was able to turn it into a mist.
In my experience with every betta I've owned, they fought the current constantly with no control valve, and this is the softest bubbles I could find. I use two in all mine. Turn it off for a few hours and see if he acts better.
72 is freezing and while you may not want it super high, it needs to be at minimum 76, but ideally at least 78. I don't agree leaving him freezing will help.
I would cut back to two or three changes a week - one or two 50% where you leave him int eh tank and one 100% where you scoop him with a solo type cup and thoroughly rinse everything, especially the gravel to remove debris.
You need to use his in tank thermometer to match the tap and get the exact same temp during a water change. Then the water should be premixed (you can use gallon water jugs from the grocery store that are rinsed in hot water only and no chems if you have nothing else) with conditioner before your fish ever feels the water. For the 100% I would also use an acclimation period of 1 hour, while the fish floats inside his cup in the cleaned tank while you add a couple tablespoons of new water every 10 minutes. Try to let as little of the cup water back into the tank as possible while releasing. Make sure the cup is opaque as well.
If you haven't been matching temp or providing acclimation during huge changes this could be the cause of your problems.
Was he always getting this many water changes? If not, what were you doing before?