At the moment he sees his mum go out to work, for which she needs to do to get this thing called money, and we need money to buy things like food, toys and things.
I think that's kind of good enough for now, and he seems satisfied by that.
Discussion
An honest answer, "this is money. It isn't the only money, there are different monies all over the world but here we mostly use this. When we want something, people want money in return and we get our money by going to work. It is how we could buy this house, this sofa etc."
Can't imagine getting into the "why" loop after that.