Also of note: from here on I will assume you've watched the episode, as my curt, poorly integrated synopses were incredibly dry to write and (I imagine) read.
Episode 3 has always been confusing to me. Only after watching it for the fifth time do I really understand how the cure in the story worked, and if you ask me about it a couple weeks from now I probably won't be able to tell you. Fortunately, the other elements of the story are a little less convoluted. I think.
|. . .What do you know that I don't, ma'am?|
The thing that held my interest the most in this episode was the concept of identification. Maho, the elder's grandson, stands out from the rest of the people in his village. While they too suffer from mushi problems, his is unique in that it makes him hear sounds no one else can. Further (and much more physically) cementing his status as an exception is the fact that he grew horns on his forehead because of this illness.
This difference makes his identity somewhat confusing. He is one of them, but at the same time isn't. Given that he seems to be somewhere around ten, it would be surprising if this didn't have some effect on his life, and indeed it has – he has apparently been hiding it for a year now. On the surface, this might look like the full story. A boy becomes aloof due to a problem, his problem is fixed, and then he gets better. There's one person that complicates matters beyond this, however: Maho's mother, who died the previous winter of the same disease.
This puts his problems in a whole new light. Maho's illness sets him apart from the other villagers, but it also very tangibly connects him to his mother. It's something he very clearly shares with her, and only her. (Here it's worth mentioning that it's probably the exact same mushi infecting Maho that was afflicting his mother, given he got it right after she, its host, died.) As such, finding a cure, for him, is much the same as letting go. The analogy isn't too hard to understand; staying caught up in the past can be comforting even if it's painful, but we have to move on to survive (every mentioned case of Maho's illness has resulted in death). Also of note is that when Maho is finally cured, he is able to finally remember his mother's last words to him (which were previously forgotten, perhaps in an attempt to forget that she's gone).
This interpretation is the only one I've been able to make sense of the last scene with. Keeping the horns after they've fallen off is his way of gradually letting go. He's now like the rest of the villagers (a normal, uninfected human), but he hasn't totally abandoned the identity only he and his mother shared. Again, the analogy isn't terribly esoteric; you don't have to forget a loved one all at once - it's okay to take time.
Another post on this episode was written by Ephemeral Dreamer early last year. I highly recommend you give it a read here.