This sounds like my ds, who spoke no more than 15 words till two weeks before his 2nd birthday. Then one day there was an explosion of words, which continued till I counted 85, I think, two weeks later on his birthday.
I remember worrying before the explosion, as his active vocabulary wasn't keeping pace with what the baby books said he should be doing. But he was fine - seven months later he was asking things like 'What would happen if the earth doesn't [sic] revolve on its axis?'. In retrospect I think it was probably a manifestation of his perfectionism -- wanting to wait to try something till he was sure he was good at it.
Maybe that's the way it will be with DS1.9. If I recall correctly, according to my mother, I didn't speak until about age 3. It makes me wonder where DS5 got his early verbal development from, if it's really driven heavily by genetics. As far as I know, my wife wasn't super-fast in that area, and neither were members of her family.
With every passing year I become more and more skeptical of heavy reliance on milestones, especially verbal / reading ones, as major predictors of intelligence. I suppose I may get less skeptical if DS1.9 turns out to be a dunce.