I had actually talked to a close friend, and fellow Betta keeper about your issue. and I also feel I owe you abit of an apology. I am apologizing first, for sounding presumptious about "adding plants, betta logs, caves etc to alleviate boredom" you just hadn't mentioned he had them, so I thought I'd chip in, but looking back, that could sound kinda jerky. not my intent if it came across that way, so if it did, I apologize, truly.
Now to business. The friend I had spoke to let me know about that the lighting of the room/vs aquarium light can make a difference, and this confirms what another member had said. Do you indeed have one of those scenery backdrops? It may help as well. She also told me, sometimes, if you move a Betta from one size territory to another, it can cause stress. so basicly all she told me to add, seems added to. but I am just trying to confirm that alot of the solutions seem go. If nothing works, you just have a feisty guy! I wish the best for him, and his finrot, and getting better.