He *was* entertaining himself at school - by chatting with the other children.
I know that even I, in my forties, would be restless and bratty if I had to sit in a desk and do *nothing* for long stretches. What exactly do you expect him do to entertain himself that would not disrupt the classroom and that would not involve having additional materials that he could engage with quietly in his seat?
When I am bored, I read a book, or write, or draw, or listen to music and sing along, or run, or do yoga, or find someone to talk to. Some of these are acceptable in a classroom, some are not. Making sure that it is easier for him to choose non-disruptive ways of handling his boredom than disruptive ones seems to me to be the least that the adults in his life can do at this point to teach him how to handle boredom without disrupting class or distracting others.
The greater question that looms here, of course, is why providing him with the appropriate level of school work is up to you, rather than something the school should be doing anyway.
Intellectual stimulation is food for the mind. Understimulation is intellectual hunger. We all know that children get bratty when their bodies are hungry - the same goes for their minds. You wouldn't refuse to feed your child if he was genuinely hungry unless there was a really good reason that food was not permitted or available, like impending fasting bloodwork,for example. Sending him to school knowing that the work is not adequate to meet his intellectual hunger and doing nothing to supplement it would be like sending him to lunch with nothing but a few celery sticks in his lunch box.
