Since it isn't making the sound when you pedal I'd say that it's just a really rough soundng freewheel. If this is the only thing wrong with the bike then I'd say you're lucky. It likely won't fail. But if it does or if he gets fed up with the funky sound take the wheel off and take it into a real bike shop and have them install a replacement freewheel. They run around $10 to $15 for a decent one and will likely cost around $8 to $10 to install on the wheel if the wheel is off the bike when it comes in. Labour will be more if you just bring in the bike.