What are my child's rights? Can I counter charge for bullying & harrment?

My 13 yr old son has a range of learning disabilities (Aspergers, ADHD etc). I had a visit from the police who said they had received a complaint of common ault on a 6 yr old for my son pushing him in a bush. The young boy was being provoked by his older brother and punched my son. My son retaliated by pushing him (witnesses agree to this). Apparently the older boy also hit my son. The little boy is unhurt. The police have said that this will stay on his record until he is 17. Also, the mother is verbally abusive towards my son; shouting, swearing - very aggressive and threatening even though I have explained my sons' problems on numerous occasions and told her how much this behaviour upsets him & 'frazzles' his mind due to the Aspergers (autism). I agree my son should not have hit back but he does not understand that is not acceptable to hit out at a 6 yr old; he thinks if they hit him he should hit back. My son is not a criminal and has never been in trouble. Please help!