A beef tweak. That sounds painful. I don't think I'd want that for him either. I know a lot of members here have bettas that flare at their own reflection, but I'm not sure how they deal with it, or if it forces them to keep the light off. If his behavior remains constant with the light on, I guess leave it off until you get his to-be tank mate established on the other half. Having a brother across the way might keep him more distracted.. or just give him something else to flare at. Either way, I wouldn't let him keep you from keeping the light off. That would suck.
Maybe, if you're able, move the tank to a place where it's not so dark around it. If the only light around the tank is the light above (or in) it then you'll be dealing with inner reflections a lot more. A place with more ambient lighting should cut back on the issue.