Try re-decorating and maybe adding a mirror for him. I used one of those compact mirrors for mine, and that keep mine entertained.
My betta used to tail bite so so much, to the point he hardly had a tail. But now I've put more caves in (constructed out of rocks) and some more soft plants as well as some tetras, and made sure the water was appropriate.
I put in more caves as it added more places of "rest". My betta seemed stressed/bored about something clearly because of the tail biting, so by giving him places to rest he feels more secure and literally rests there. I gave him softer and fuller plants, and in addition to the cave this gave his tank a bit of redecorating which interested him. I reccomend every time you do a proper water change to switch plants and rocks around (AKA redecorate) as this peaks his interests.
You mentioned the pet shop people told you the tetras bit his tail, which seems very odd as tetras are very docile. I think the pet store may have lied to you about that but don't quote me on that. The tetras are also a "distraction" fish. Mine loves to chase them around but there hasn't been a full out war or fight, he just likes to protect his territory. You don't need a lot, six would do.
As for the water, just keep it warm and clean to promote healing. An ideal temp would be about 81 F.
Oh and the mirror! I added a little compact mirror into the tank to keep him occupied as well. When he sees his reflection he flares and it's meant to be great exercise for them. But I remove it as well when he is litterally spending his whole time in front of it, as too much flaring can also stress them. A little bit is good, too much (maybe after 10 minutes a day) is bad.
This is what saved my fish from tailbiting! To this day he still hasn't and it's been months!
I hope this helped you! Good luck!
