The teacher had him assessed and he was diagnosed with ADHD and Autism.
But that really isn't relevant, and here is why. He is nearly 7, and has just been dignosed this year. When he was three, I had not an inkling that something was 'wrong' and to make him sit still the way ds2 will sit still, I would have had to physically beat him unconscious. I didn't know about the ADHD, I didn't know about the Autism. NOBODY did. He was just the child that couldn't sit still. To all eyes, he looked like he'd never seen a table and chairs in his life, and to some extent still does.
Knowing the diagnosis doesn't change his behavior. He has always behaved that way. He probably, despite the best interventions, always will.
When he was three, all the training I did was entirely pointless. I might as well have tried to teach a cat to type. he cannot sit still, and once I accepted that(6 months ago), and stopped trying to discipline, 'tone', persuade and 'consequence' it into him, life got easier. Now, if he is fidgeting too much, we simply leave.