Do you think Doni is nonverbal because he's autistic?
This is the second Doni ask I've received. Are you the same anon? I'm working on the other one, by the way. 😁
He certainly could be autistic, but personally I think he's traumatised. Zevlor and the other tieflings mention they're refugees and survivors from Elturel. Without going too in-depth, Elturel was a major city in the Western Heartlands of West Faerûn. Zariel, Archdutchess of Avernus, pulled the whole city into the hells, specifically into the River Styx. If Doni witnessed it while escaping, he would've seen something like this:
Terrifying. The city was sucked through a black hole, leaving nothing but a crater behind, and all the survivors were turned into devils to serve in the Blood War. This easily could have caused Doni's mustism, and if that is the case, I hope his voice recovers. It also explains why he put letters in rivers and addressed them to the Styx, hoping they would reach his father.
If anything, I'd argue he has hyperlexia, which is often (though not always) part of the autism spectrum. His writing is impeccable, especially in comparison to some of the other letters you can find. Mirkon's letter, for example, is typical for a boy his age; full of errors and small words. Though I personally think Doni's mutism is the result of trauma, he could very well be autistic, too.