Heh, I think he needs to be explained how just long a period of time 3.75 billion years is. He knows division and multiplication? You could try to explain it to him using a comparison of a ratio of distances anyone can understand. For example, the the 1000 km or so distance to your relatives in Lapland is about one billion millimetres. An insect lover, I'm sure Miro knows many insects that are about one millimetre in size. You could tell him that one billion is such a large number that you must have a billion of those 1 mm sized insects to form a line from your home to Lapland.