Tom, the first thing you need to do is measure his head and if you can try a helmet on. We sell quite a few helmets at Acceleration Kart Racing and most 12yr olds fit into an adult helmet. Make sure the helmet you buy is Snell 2010 or 2015 approved and some organizations require an FIA CMR rating. There is also an ECE rating that I think applies to motorcycle helmets. A good kart shop can set you up with a helmet that suits your needs. Feel free to call Acceleration at 888-466-5278.