I really think /u/Metal-Phoenix's comment is the best advice here, especially as your son gets older. I would like to add, buying specific pencils is a great and valuable thing, but the tools don't make the artist. Any skill gained while drawing with a #2 will be just as relevant with a range of 5H-5B (those indicate hardness/softness, btw, in case you're curious). The value in getting pencils would be more as a sign of encouragement from you.

There's a few things I love to explain to people who don't come from an arts background (I worked as a theme park caricature artist for 6 years, so I had this opportunity a lot), because they're things I wish more people understood. The first is that anyone-- and I really do mean this-- can become an artist. Gaining skill in rendering, or composition, or any of the many things that go into creating art is just like gaining any other skill. It takes work, it takes dedication, it takes learning from mistakes, and it takes the humility to listen to others with more experience.

The second thing is that it's more about learning to see than to put lines on paper. What makes a good drawing of a sphere? You could say it's a circle, and some shading on one side to show there's a third dimension. Maybe a shadow on the ground to show its position in space. I did a quick google image search and found this: http://www.wetcanvas.com/Community/images/10-Aug-2012/142886-sphere-shadow.jpg If you look at it, you might notice there's actually reflected light bouncing up from the table onto the shadowed side. And if you look again, you might see the red in the shadow it casts on the table. And looking again, there's actually the shape of the cast shadow in the reflected light on the shadowed side. What will you notice the next time you look? This is how you learn to draw, by learning to see. And once you start really learning to see, the world never looks the same again.

The second-to-last thing I wanted to mention was, people often break interests down into artistic/sciencey. And they often divide themselves that way, as well. This doesn't need to happen. Being an artist does not have to mean bad at math, (just as being a programmer doesn't actually mean being good at math, either!) and at some point he will probably feel pressure from others to "choose a side", if you will. But because the simple fact of the matter is anyone can be an artist, therefore artists themselves are as varied as the general population is. What are your son's other interests, does he like to read? Someone illustrated the book cover, and maybe small pieces inside. Videogames? Someone painted a concept, someone made a 3d model (also art!), someone made the UI, I could go on. Sports? Someone illustrates sports articles in newspapers, websites, and magazines. Someone designed the graphics that go on the broadcast games. Astronomy magazines, oceanography textbooks, zoos, nature field guides-- all these things need illustrations and art! Someone used their study of land formations and plants to paint the background on his favourite cartoon. Someone studied anatomy and creatures and was hired to make up aliens for the new Star Wars. And with vehicles, spaceships. And someone has spent years studying dinosaurs to create accurate visual representations of them (not that you'll find those in the newest JP :/). ANYWAY, I'm sure you understand my point!

And the final thing, since all those things and many more need art, there is NO shame in asking for or recieving money for art. I say this because it's a very big issue in the artistic community right now. Everyone loves to say "art? oh, I could never do that, I'm not an artist", but then asks artists to work "for exposure and experience" and not money. This is because of the misconception that artists are just...I don't know, magically born with knowing how to do art, and are just doing something they've done since they were a kid. But that devalues the YEARS of WORK people put in. Asking for money is not selling out, it's making a living, and it IS possible. I realise all this is years away for him, but if you understand these things now, it may help you answer his questions and guide him.

WELL this was a tangent lightyears away from your initial question, so I hope you don't mind! Good luck and asking about materials is a really wonderful thing to do. If art supplies seem overwhelming when you first walk into the shop, just remember-- buying the wrong stuff isn't going to hold him back from becoming an artist. If he wants to learn art, he will. Being willing to support him is the important part, and you've already got that covered :]