Dandruff - dry skin particles that fall off of the scalp.
Given this definition, then I'd say bald people do get dandruff, if their scalp is dry. It's just less likely that their scalp would dry out because it is exposed to the air. It's possible that the stuff your son had was dandruff, or maybe it was something else. I don't know. I'm not a doctor. I'm just smart (sometimes).
My sources tell me that bald scalps are oily, and dandruff is dry skin, oily skin isn't dry, therefore bald people don't get dandruff? Or something like that. To make sure I am right, I propose a random sampling of bald people to measure their scalps level of oilyness.