Depends on the kid really - would probably try to keep this information from him until he is nearer 18 - but that would probably prove difficult if all his cousins know about it - unless all the parents decide the same course of action. He will probably have matured quite a bit by then and maybe you won't have the same worries.
Do you think he'll go to third level education. It also depends what he wants to do with his life - maybe he will be happy to have a little splurge (like a fancy laptop, little car, new bike or whatever) and then put the rest in a deposit account for a few years.
Think at the end of the day you'll probably have to sit down and try and negotiate a good plan for this money and try to get across to him how lucky he is!