Jon wasn't planning on fixing Sarah, at least not yet. But he was beginning to get an idea about how to fix himself. He remembered how it was possible to reinterpret a wish on that tree branch... if you just wished for it to be blue, you could wish it was a better kind of blue.
So he'd need to figure out exactly why people think he's a girl, and make sure it's something that wouldn't be too much of a problem.