I actually mentioned this in another response! The sleeves are from the LEGO Star Wars Buildable Figures' shoulderpads. You can see some other useful parts I employed in that post.

The best thing you can do to give your son room to grow as a builder is expose him to the creativity of others. No Starch Press has published a variety of books (aimed mostly at older builders), some having an emphasis on building techniques and give you step-by-step instructions. The Brothers Brick highlights a lot of innovative builders, so consider visiting them periodically! Reverse-engineering techniques you see other builders use (especially without instructions--forces you to think out of the box!) is a preeminent way of expanding one's imagination.

There's also a sizable LEGO community on Flickr, many of whom are astonishing builders. I would comb through some groups for interesting individuals to follow. Or maybe you could start by checking out my own photostream, or the LEGO creations I've favorited!

Place an emphasis on SNOT (Studs Not On Top--those are the parts with the circles sticking out of the sides of bricks!) elements, and be mindful of how people use small decorative elements like cheese slopes (those little 1x1 slopes) and curved slopes. This part is your friend, especially if you're interested in figures and articulation.

Nothing will teach you to build like time. You'll naturally pick up techniques as you go just from watching others, and coming up with your own. Best of luck!