Well, if he was 12 or 13, then it would be teaching him a lesson perhaps, but at 7 years old, he is just practically a baby still, so no, I don't think it's right to handcuff babies to try to teach them "lessons." There are better and more appropriate ways to teach lessons at that age.
They learn at a very simple level. "Violent misbehavior = handcuffs and embarrassment, or worse".
We live in a violent age, with few disciplinary actions taken by the schools. Ergo, others must act.
Hell, I got whupped on the back of the legs with a map pointer in class at that age just for not knowing my multiplication tables forwards and backwards.
You can bet I knew them the next day.