It'll probably take some time for those fins to grow back, considering the fact that the infection had eaten all he way into his tail meat. He has to rebuild the flesh first. Any animal's body is going to heal according to priority over appearance. So the muscle and skin and scale first, then the fins.
And don't expect them to grow back as pretty. I know when my fish lost some finnage due to me trying to mate him with a very aggressive female, some of the rays of his fins never grew back out completely, which gave his fins a crumpled look. It didn't hinder his swimming and he still had a nice veil tail, but it was never quite as lovely as it had been. But sometimes function serves better than fanciness, right? And he seems like he's gotten real sweet on you through this whole ordeal. ^__^