Michelle my plan is to introduce it gradually - in primary school he can do a mini-fast where he fasts for just a few hours before Iftar (the meal at the end of the fast) and then when he gets older he can choose when he's ready to fast a full day. I wouldn't expect him to fast the whole month of Ramadan until he reaches adulthood but if he wants to when he's a teenager I wouldn't try and stop him.
But Ramadan is about more than the physical dimension of fasting - you're also expected to try to be a better person who argues less and does less backstabbing etc so I'm hoping that I can start introducing those concepts a bit younger.