Can a gifted kid has ADHD at the same time without behavioral issues?

MY DS (16) is identified as gifted (WISC IV 143, tested at 7th grade). He has been a typical good student (meaning that, good grade, no behavioral issues, following the instructions well in any setting, a role model in the class or clubs, and etc). He was bored at school but he managed okay.
However, when he hits the high school, I've noticed that he took so much time to finish his school work unnecessarily. Frequently, he was distracted and started spending time in googling or doodling in his notebook during homework, or he simply looked like doing nothing on his desk. He said it's hard to focus on his homework (especially writing homework).
His grade started dropping in some AP classes. He was also stressed out and depressed. We visited local psy. The doctor talked with him for 30 min. and said he has ADHD as well as depression.
I wasn't surprised by depression becauase he has been unhappy since we moved to the new town in 9th grade, but ADHD? I never observed well-known ADHD indicators when he was young such as attention problem, behavior issues.
I thought his current attention problem came from the stress or depression.
Does anybody know the case of ADHD without much symptoms?