Shoot, I was hoping you'd say I was worrying for nothing.

CCN, I'm hoping that since your DS took a similar path that it will prove to be nothing. I might give it a few more weeks or so to see if it improves or disappears.
i'll just chime in with my dad's favourite story about me as a child. apparently i said NOTHING for 2 years - and then one day in the car, we passed a construction site, and i said, "look, mum, look at the crane!" dad apparently nearly drove off the road, pulled over and then everyone had a good cry.
they told me i also repeated everything they said for a time, as well - but they eventually worked out that i was actually mirroring THEIR pattern. every time i said something, they would repeat it to me... so i started reflecting that behaviour back to them - until i realized that wasn't the way everyone communicated - just my weirdo parents!
so... it totally could be something as silly as this! but everyone's advice above, is as usual, totally great. all the best to you!