After we read this book on counting by tens, i continued the lesson by using the Montessori method
Using the tens board and beads, i showed N that two tens make twenty, three tens make thirty etc. My presentation must have been a little shaky, N didn't seem to get it. But fortunately, an inspiration hit me, he seems to be able grasp the concept.
After the presentation, N got to practise what he learnt. I'd call out a number and N would bring to me the said number of beads. For instance, i would say '30' and he would pick up 3 tens and hand them to me.