Your lights shouldn't be causing that much fluctuation. Do you have a heater that you can adjust? If so, it should turn itself off as the lights heat the water, so that your temp remains roughly the same.
Also, since bettas like lower light, how is it working with your current lighting? Do you give him lots of places to get out of the light, like caves, rock shelves, plants with wide leaves? If not, you might discover he is happier - more active and interactive with you - with less light, places to hide, and steady temp.
What was your temp before and after the change? Bettas need water in the 78 to 80 range, and are OK up to 82 - 84. They may be OK with a sudden change of 2 degrees, but will really do much better if the water temp remains steady or changes gradually. The best way to achieve this is to make sure the replacement water is the same as the water in the rest of the tank before you put it in. You can do this with a thermometer, but it is easier just to use your hand - stick your hand in the tank, then in the replacement water. If it feels cooler than the tank, add some warm water till it feels the same. If warmer, add some warm water. If you removed the betta and some of the water during the change, the temp needs to match the water the fish is in, even if it has cooled somewhat. When you add him and the water he is in back to the tank, the temp will return to normal, assuming you haven't changed your heater setting.
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