It may well be that your child is good at math but not particularly interested, a perfectly valid choice. However, I would never assume someone doesn't like math just because they don't like the tedious, shallow, repetitive calculations that pass for math in most curricula. If you have a chance to expose him to some real math - i.e. taught by mathematicians, because they love it - it would be great for him to have the opportunity to see what math really is, and whether he likes that more. AoPS is a great source (for both courses and personal dabbling, as Portia mentions) but he could also just play with NumberPhile videos, Martin Gardner books, etc to see if he has a taste for it.
