Indeed.
Part of the issue here is the definition of ‘non-verbal’.
Both of mine were late talkers. Said pretty much nothing at all until 3, and only a vocab of a few words at 3.5. Then had speech/pronunciation issues that meant they had to have speech therapy to learn how to create the sounds needed to be intelligible until they started school. One was regarded special needs on application to infant school on the basis of his speech needs. We only later (in teens) received autism diagnoses and then ADHD.
I had an identical path to speech - massive lisp and sounded like Donald duck apparently. But all of us got there by 6 or 7 and, essentially, haven’t really shut up since. With my eldest we joked that she either didn’t feel she had anything to say to us or that we were too uninteresting/unworthy to warrant her time. She spoke once she felt she had something to say and once she had someone she deemed worthy to listen.
I think without a real explanation about what his ‘mutism’ or ‘non-verbal’ issue was, ideally the actual clinical diagnosis, we can’t really hope to understand the extent of his issue or the level of achievement in overcoming it.
It’s all smoke and mirrors until then.