Can't fathom how kids can be interested or not in things they have no experience with.
Big brother-esque/authoritarian programs such as this which enforce arbitrary learning boundaries kill the natural curiosity and spontaneity of the child because it forces them into a mold rather than fostering a genuine interest.
It is very observable among kids. They try to learn everything by observing around them. Limiting the window to which they can learn is what the conventional school system has implemented through ages.
I cannot find the point in forcing a kid to learn math when he is clearly interested in music or art. At best he will become a bad mathematician. Where he could have become a great musician. The idea of forcefully planting interest of something inside a young mind is terrible.
Instead, give him the freedom to choose. Give his both math and art. Let him choose what he likes.
And please don't give a reward if he chooses math over music.