The fin looks weird. What should I do with the betta?

I had this betta since last August. He’s well until he recently got some ich - white dots. But we’ve put some anti-ich drop, increase the tank temperature, and it seems there’s no more white dots on his body.
But then after a few days, he’s fin start to become unhealthy, shown as picture.

What can I do to make him recover?

I first recommend to stop using the ich stuff. At this point, it’s probably making it worse.

Fin rot can be treated easily. 75%+ daily water changes per day, for a week usually clears up fin rot. Added aquarium salt can also reduce the chance of infection. The same treatment above can help solve a few of the common injuries including missing scales and torn fins. Weekly water changes after the fin rot has disappeared will help prevent this in the future.

(https://www.fishforums.net/threads/betta-research-paper.465208/)

(Also, according to the picture, your pH is about 8.2, that’s a little high. Are you sure you did the test correctly? Did you test for the regular pH? (Not the High Range pH))

What are your tank readings? Especially Ammonia? Try daily water changes for a week or two and add some salt and see if that improves it.

Add some Indian almond leaves for good measure. They'll soften the water and reduce stress, and they have mild antibacterial properties. Daily baths in aquarium salt (NOT table or epsom) should help too, ideally in a separate container. You might also want to start looking around for some fin rot medication or a broad spectrum antifungal or antibacterial treatment, just in case it persists or gets worse. What are the ammonia, nitrite and nitrate levels? The pH is on the high side, but it's more important that it stays stable. Chasing pH with chemical additives etc. is typically worse & more stressful to the fish than keeping it stable, even if it's a little out of their range. So unless you can afford RO water or an RO system, there probably isn't much you can do about it. You can try adding driftwood or other natural sources of tannins to make the water a bit more acidic.