We’ve got a safe t seat http://www.safe-t-seat.co.uk/index.html
its great! It means its very easy to know how they’re doing, you can talk to them and see whether they are enjoying themselves without having to turn around. They can see where their going and become much more a part of the ride, than if stuck out on the back behind you. My daughter loves it and rocks from side to side if I’m not going fast enough.
fitting is easy, you just need 10mm of spacers between headset and stem.
EDIT found a pic
2nd EDIT make sure your son has enough strength in his neck to hold it up with a helmet on. we had to wait past 12 months until we were confident in her wearing it for any length of time.