I agree with finding a different style of chair for him such as a stool, a chair with a footrest, or even a beanbag chair. My son's not big enough to lean back on the dining chairs yet since his feet don't touch the floor, but he's already broken one anyway. He was kneeling on the seat, talking to me through the back of the chair while I fixed dinner, and the chair fell over and the back came completely off. My son(who has ADHD/Aspergers) also has a lot of nervous habits that he doesn't even know he's doing. A piece of paper on the floor becomes a skate. He chews his shirt collar, and constantly hums and sings. The difference on days he take ADHD meds is dramatic. I usually try to redirect the behavior to something more appropriate, such as chewing gum or getting exercise. I talk to him about stopping the behaviors when he is able to so it doesn't become a bad habit he's stuck with forever. If your son is old enough, you can involve him in brainstorming solutions for this. Whether it's a different chair, a hand signal instead of verbal reminder, a reward or punishment system, going for a quick walk or stepping away from the table for a minute, he might be more likely to follow the plan if he helps come up with it.