I've wondered this too!
My mom's family is FILLED with musicians, artists and other creative people. My husband was a sculptor/painter before he went into web development/design. My father is a drummer, a very good one! I draw and write all the time. My husband can draw something that is real (like a bird) but I'm good at abstract drawings and he isn't.
My son, who is now 15 months, can drum already on a 3 piece kit, pick up the beat of any music playing to dance to it, and tries to sing. He seems to love drawing already - he demands to 'draw' whenever he sees me with my sketchbook, so I let him. I also let him watch me, so he can see the craft being practiced, as well as us drawing together, with me drawing circles and other shapes.
I'm more of a scientific person, and wonder how on earth these sorts of things get passed down genetically, and agree with the person who says basically that the brain mapping to these things get passed down, but it takes it being nurtured to truly bring it out. Some of us seem 'programmed' for math or music for example, and developing that skill is just easier for them. I think we ALL can do these things, but some people will just naturally have brains that have it make sense and have it WORK for them!
Give your son some crayons and let him draw away - there is a good chance he might be good at it!
And I'm glad to know I'm not the only one pondering these things!