People become interested and disinterested in things all the time. No child should be forced to study anything. People don't learn what they're not interested in. Not all people are interested in the same things, and that's okay. Not all people have to know the same things.
Variety is important.
A society full of people who are forced to learn the same things is boring and inflexible. If your son wants to become a doctor, he'll still be able to, even without grade 8 math or grade 11 science. People who can work on their own without constant supervision are always in demand, as are people who have unique ideas and opinions, as well as the initiative and skills to use them well. You don't need the help of teachers or the backing of a curriculum to guide your children through to adulthood. Although we are a society of people who turn to outside help at the smallest hint of difficulty, it isn't necessary.