My husband has ADD (not ADHD, he's not hyper active). He was diagnosed since he was 8 years old (in the US, not HK). He took medication when he was in school, but that medication didn't help at all... he was like a zombie.
He's on another medication now and it really helps him concentrate, and I can tell a difference.
So there is no one magic medication that helps everyone. You'd need to try to find the right one that works, with the right dosage, without side effects (he doesn't really have any side effects with this one).
if you try one brand of medication and it doesn't work, you can always stop, and try another. You won't know if it works or not until you try.
Think about it this way, when your son has other medical problems, like a fever or a cough, or whatever, do you ever question the doctor for suggestion taking medication (even if it's just panadol?)