1) Cut out pictures of little pigs and houses (stands for 100 and villages (stands for hundreds)
2) Ask him to do addition by putting groups of pigs together e.g. 5 +3
3) When the sum goes up to 10, I'll ask him to "trade" 10 pigs with me for a house - that stands for the tenth
4) Similarly, when the sum goes up to 100, I'll ask him to trade 10 houses with me for a village
Furthermore, I'll use to ask him to think of way of making up a number, e.g. say 11, he will be asked to think of any ways to represent this number, such as