nothing much he can do. most thing he can is upright apologize. retconning, rewriting or giving a different ending, they wont cut it. like many have said; it wouldn't be enough and would just make the other side angry. he's stuck in a no win situation. the damage has been done, all we've got are fanfiction/fanart now.
I hate to admit it but that's unfortunately what its come down to. Its a no win scenario, but he can at least wipe some of that bad if he comes out and apologizes or even admits his wrong doing to his fans. Not just a run around like when he admitted to being bad at romance yet never admits his canon couples were poorly written, even going as far as to acting like the " red herring" was misunderstood and we the fans are just reading it wrong.