ADHD comes in various subtypes: primarily inattentive, primarily hyperactive/impulsive, and combined. The symptoms also may not be that evident when engaged in high-interest activities (classically, video games, favored sports, etc.) Keep in mind also that the disorder is not really a deficit in attention (name of the syndrome notwithstanding), but a deficit in regulation of attention. So over focusing on one thing can be as much a symptom as flitting from one thing to another can be.
It sounds like he is engaged by creative work, but not by nitty gritty fine tuning work. And probably hasn't yet learned that there can be quite a bit of creativity in editing and revising as well.
Typically, ADHD is considered to be an intrinsic profile that exists from early childhood (formerly, before age 7, currently, before age 12), but environmental demands may not expose the profile until later.
