Due to the age of the bully, it is the onus of the victim to prove malicious intent rather than an accident. His parents can claim "kids will be kids", have their defences up and the police may not dig without a written backstory that the school agrees with. And due to the age, the police may still do nothing.

Although you can report the school for not making a note of the incident (the child was told it would be written down "next time" - beggers belief that) as it was a serious "accident" that caused your son harm.