My betta seems to get bored easy too, I think it's because he's so smart
lol ( I'm such a proud mommy) Anyway, I put different things next to his tank every day, I just walk around the house and pick up whatever, knick knacks, colorfull pictures, statues of cats, sometimes just because it's colorful I'll stand up the comic page from the newspaper up against his tank for 20 mins or so. He's so cute when he comes to check it all out. Sometimes when he's really board I just type in "videos of female bettas" on my tablet and stand it up next to the tank and let him watch all the pretty ladies swimming around, he loves that.
Anyway, just some ideas if it is tail biting because he's bored. If it's stress, have you checked the outflow of your filter? If there's too much current that may stress them too. Just a thought. Google betta filter baffel, there's a good post on how to make one from a water bottle. I did it and it works great.
Side note...until you figure out if it is tail biting, and if so if it's from borredum or stress, you might not want to do the mirror thing, it might stress him more. If it's bordum it may be a good idea but not if it's stress. Just my 2 cents.