Recently N is into counting aloud. He would count from 1 all the way to 100- without skipping a number.
He went on with this counting spree for days but would stop at 100 (because he didn't know that there were numbers beyond). That's when i decided he was ready to learn numbers after 100. During his next counting spree, when he reached 100, i continued on by chanting 101, 102, 103 etc. As i had expected, he was hooked and continued chanting all the way to 199 and stopped again. That's when i introduced the number 200. There were some mornings where he'd wake up and count from 1 all the way to 200 over continously (while my hub and i were still struggling to get out from bed. And if you didn't know better, you'd think he was counting how long it would take us to wake up and get on with our mornings. Hee hee).
And that was also when i decided to teach him to count by hundreds. So we counted aloud, 100, 200, 300, 400, 500, 600, 700, 800, 900 to 1000. (And as i am typing this, he is getting the visuals on these numbers, reading them aloud as they appear on screen. You see, i have no energy to give him proper lessons, so im just going to take any chance i have!). Just to make sure, he understood that there were numbers between these numbers, i would ask him what number is after 303 or 452 etc, he could give me the correct answers. And that's how I taught him higher numbers verbally without using any visual aids.