I just want to say that I agree with what's been said about not worrying. If he has plenty of decorations and hiding spots, he should settle down after a bit. My guy did something similar when discovering a reflective corner in his tank. Every now and again he'll go back to the spot and flare up, but he's not excessively obsessed with the "other betta."
Give it some time to see if he settles down. If it continues for a long period of time, then consider putting in decorations, hiding spots, and maybe covering an area of the tank.