Dear Mavis,
I hope you don't mind me jumping in here. I think it is wonderful that you son can say two ten ten. That shows he knows exactly how much it is. Remember, he is only four. Language is still new for him. He has the conceptual understanding and the language part will solidify. Please do have him use the abacus to show you the answer and stay away from relying on the paper and pencil and "you" writing it out for him. We want to enable our children to go beyond us in mathematical reasoning, that is why visualizing the abacus in his head is so important. That is what the Japanese do that causes them to surpass us in mathematical reasoning at such an early age. Keep pressing on. You are giving your son an incredible advantage by starting so young, but be sure to stop and play the games. The matching quantities is critical for him to be able to start "seeing it in his head" too.
Hope this helps.
Nancy

Hi Nancy,
Thanks for your kind advice. I usually write those equations on the board for him to solve (mentally). But at times like this, i do try to ask him to use the abacus if he doesn't get it- just that he's sometimes unwilling to do so. Yeah, guess i'll have to encourage him to use the abacus more often so that he can picture it in his head : )

Many thanks!