Ahh, OP, your poor boy. I'm sorry to hear how he got to a crisis point. And how horrible for you, too.
I'm relieved to hear how swiftly he got some intervention via the Crisis Team.
I have a DD who was selectively mute until she was 6. She was v bright, curious, quirky. Diagnosed, "Aspergers" at 13...she is now 29, I had no idea than selective mutism was so strongly associated with Autism, but these were words you used. Also the tics, as other PPs have mentioned, is highly correlated with Autism.
My DD ended up reading Chinese at Cambridge, going into teaching, leaving, as was disillusioned with the tight, prescribed curriculum, and is now working in an organisation that helps schools teach critical thinking, and how to recognise dis/mis information.
I questioned how helpful it was to get a diagnosis, ( had considered ND for a long time before pursuing an assessment). But I am very glad I did. What triggered me pursuing it, was a general decline in mental health...she became depressed, almost selectively mute again. I can't say it was a crisis, but it was a slow, steady decline.
She is largely happy now. In a happy relationship and recently went back to the school where she taught to present a workshop on critical-thinking. To great "whoops". It was a happy moment.
My point is, in your poor old boy, there are, indeed, some ND traits, and the crisis Team are good in picking this up.
Have you thought how hard he might have been working to "mask" this, and how it got to the point where he could not fight any longer....?
I wish you so much luck, this must have been truly horrible... for you, and he, and any other family. X