He needs to stay rear facing until he is 4 years old.
Don't be too sure. From the age of 1 or so we spent our journeys chatting to our kids about what we can see out of the windows. I reckon this is pretty important for their development, we now have two great travellers (aged 2 and 4) who love to chat to us. I think being included in the family is more important than slighly improved crash safety.
When there's only two of you in the car would you sit your partner in the back facing backwards too? No, of course, not - cos you want to talk to him or her. Well, your kid wants to talk to you too.
Plus the 4 year old was 107cm and 24kg odd at age four, there wasn't a chance of being able to have her rear facing at that size. Nor was there at two years old to be honest.