I think it is a problem for any parents to make their child work hard at school; some pupils don’t want to study at all in primary school and vice versa. In my opinion, there isn’t any universal medium, but getting a child interested in his learning. It’s no use punishing him, praises can be effective only for some short time, promises of future rewards make your child idle anyway. Getting the pupil interested means to explain to him, that everyone has his own type of occupation: grown-ups go to work, and pupils learn. It’s very important to bring it home to the kid, that he studies only for himself and for his future. The teachers don’t want to hurt him, and even bad marks are good in the end. The teacher seems to be an enemy to small children, and some get this attitude towards teacher in habit even in secondary school. If you see that your child isn’t capable to work the whole day through, you can suggest his learning several subjects as essential ones, but he must receive only excellent marks, as they are inextricably connected with his future career. But he mustn’t stop learning other subjects of course.
The example of someone in the family is very important still. You can say: ”Look at your granddad, he’s an architect just because he studied at school perfectly”. The kid will be ashamed, and it’ll help him to realize that it’s very important to do well at school. The sooner he can work hard, the better.