How to extend the motivation ?

DS, we believe, is quite bright and understand things well. In our opinion, his two main areas, where he lacks are these: (1) Motivation and lack of it (2) Communication effectively what he understands.

We would love to find some "magic" on the motivation part. When he likes something, he goes all the way - not just with the cars (surprise !), but also his learning comes through clearly, in areas when he wants to apply. For example, he doesn't like math very much - my view is that, that is because, in math one is either right or wrong; and he doesn't like being wrong. So learning math is always stressful for him; he wants to check-mark it and finish whatever is in front of him in the paper. Thats the background.

His mother has instituted a process with him, where he earns money and spends money. The way he earns money is by doing things faster than the assigned time. So, if he has 20 minutes to do something, he finishes it in 15, he is 5m early - this will earn twice that - so he earns 10c. He uses this money to spend on what he likes - watching movies, taking breaks, chicken etc.

Few days ago, he was short on cash; he owed his mother some 18c, by end of the day. So, before he went to bed, he declared that he would do one of his chores in 14minutes (which has the allocated time of 20min) and will get the another chore done in 7 minutes (assigned time of 10min). That does take care of the 20c he owed. I was really surprised that he worked this out mentally - the equation I saw was 18 = (12 = [20-14] x 2) + (6 = [10 - 7] x 2)... This has mental addition, multiplication and subtraction...

So where does this motivation come from ? How do we leverage this ? Ideas ?