I'm surprised that no one has looked at this situation in context.
The boy was not handcuffed as a punishment. The boy was handcuffed as a way to stop him from harming himself and those around him after all other attempts to calm him failed.
Many parents demand that their autistic children be placed in regular classes, which is fine, however if children have outbursts, they need to be placed in classes that are equipped to deal with it, with staff that are properly trained and have the resources available to handle them.
If there was a failure here, it was not on the part of the police but either on the part of the parents for demanding the boy be placed in a class which could not properly accommodate his needs, or the school/government for placing him in a class that could not properly accommodate his needs.
If he were in one of these classes that are prepared to handle such children, and had such an outburst, he would have been restrained in either a lock hold of some type by one or two or even three individuals, or restrained using soft restraints in some manner.
The police did not act out of line, they acted in the only way they could.