Betta Fish Vs. Tea Tree Oil

I have a 10 gallon freshwater tank, that is approx. 1/2 full. I have one single male betta fish. About 2 weeks ago my daughter dumped an unknown amount of tea tree oil into our tank (it was my fault, I left it sitting open next to the fish food). I removed him immediately and kept him in a hospital tank, and did frequent water changes on him every day for a week. He has survived, however, the majority of his color is gone. He was a beautiful dark black, and now he's gray. His stress stripes are very apparent, and lots of his fins have fallen off. He's definitely improved, being active again and no more labored breathing or laying on the bottom lifeless. I'm just not sure what to do at this point to continue helping his recovery. Any help would be appreciated. Thank you!