I opened this as the last (I think) story I needed to read in hp_beholder this year, and left it open over the weekend, and so I am not sure at all who you are, Mystery Author, even though everyone else presumably knows.
ANYWAY. I wish I hadn't waited so long to read this! It is really hurt-y. Cuthbert's matter-of-fact way of looking at the world, and his confusion and consternation with how people don't tell the truth all the time is so well-described. I love how his dad looks after him, and want to swat his sister. It seems like Walburga really did save Cuthbert in so many ways, and it makes so much sense for Cuthbert to assess himself as having fallen in love with her. In his world, she is so sensible. And so trapped. And since Cuthbert clings to facts rather than hope, he can't see any way out.
The way Filius assists him is likewise so well-drawn. From what I understand from folks I've known on the autism spectrum, "learning the rules" is so helpful, and learning things like what facial expressions and non-literal statements mean are like a key to Cuthbert here. I love also how he comes to understand the reason for people telling stories that aren't strictly factual.
This is a phenomenal story, and I regret that it took me so long to give it a chance!