I think it's probably KP. It's a skin condition where tiny hairs get trapped inside the follicle and coil up, causing little bumps that are often filled with dead skin cells and oils. Yum. Did you look to see if there are bumps on his upper arms? My dad has it, I have it and now my son has it. It's not curable but it's not serious, only cosmetic. Lucky for your son, he's a boy :) The bumps on my face are gone, now that I'm an adult but they're still on my arms. I also have it mildly on my thighs, which is normal.
Here is what I know: It gets better as you get older and it gets better in the summer, when your skin gets tan. It can get worse in the winter when your skin is not as well hydrated, so use lots of moisturizer. I don't think there's anything a dermatologist can prescribe that is better than what's available otc, since you can't completely get rid of it. I've found that Eucerin Plus Intensive Repair Body Cream can help. Also, I've used a product called Glytone, available from Dermstore.com. It's a cleansing product with an acid to help clear the clogged hair follicles. It's not that cheap, but if it really matters to you and your son, it's worth it. The site offers a 10% discount on a "favorite item" and they also have free shipping. It's a nice company.
Good luck! I think it stinks to have this, but it's nice to know I'm not alone!