Yes just like LittleBlueFishlets wrote in that post that you was commenting that it stressing him out and stress affect his immune system. So if you think it the light i would try to turn it off and see if it help. How much of the natural light you have in the room. They in the living room right? So that light coming from the window should be enough. Try to play with light and see if it helps. The reflection is caused by the angle at which the light is hitting the glass. Often turning the tank or moving it to another location might fix the problem.Also , the background color outside the aquarium can enhance or reduce his ability to see his reflection. Darker colors show the reflection more than light colors. Keeping the tank away from walls or neared light colored walls will be better then keeping it against a dark wall(well i think your tanks not against the wall, sorry didn't look at the picture again) Also, attaching a scenic image to the back of the aquarium glass will often camouflage reflections as well. Most fish stores carry aquarium backdrop images you can tape to the outside of the tank. While a solid blue or black backdrop will worsen the reflections, images or aquarium plants or rocks will hide the reflection.
Silk plants is a great way to break up the sight lines between your betta and his reflection. If you buy live plants, please don't forget to quarantine them. Or let me know i will give you link where people instruct you how to disinfect the plants. Opss sorry i think you already bought plants? Sorry i read so many thread i forget.