Music/piano/keyboard is great for working the brain and improving finger motor skills. Most kids start around 4 or 5 and it usually lasts till primary/sec school... Also I found that most students in top class of pri school take music lessons..
My friends send their kids for martial arts...
My friend's kid was also diagnosed with ADHD and oppositional behavior.. His is quite severe when he doesn't want to do homework he will hit his parents. Currently undergoing therapy.
I think it's different for different kids. My kid has slight communication problem.. 2 or 3 words are fine but full sentences, he cannot do it yet. My friend's son was also diagnosed with ASD but he speaks perfectly and even has an ang moh accent which he picked up from watching youtube.