For the record, this started a few weeks ago, perhaps at 3 years ten months, or a little later.
It reminds me of when I first learned to pronounce Hangeul. I was an adult at the time, so I remember it clearly. After a month in Korea, I could reliably struggle through a word or short sentence. Riding the bus home in the evening was exhausting because I found myself trying to read each sign we passed - and street signs are horribly thick here - but almost never finishing before we had passed it. Obsessively on to the next one. And the next one...
He sometimes has trouble distinguishing between '6' and '9'.
Time to work on his actually writing them.