Respectfully, you cannot file anything because you were not the one assaulted or sprayed. Your son needs to consult with an experienced criminal defense lawyer admitted to practice law in NJ about the options to defend the charge, as well as the possibility of filing a counter-complaint as part of the defense of this matter. The charge of 'fighting' is usually graded as a PDP offense that exposes your son to a sentence of up to 30 days in jail and a fine of up to $500.00. It is an offense within the criminal code and once it is diposed of (hopefully by dismissal) he will need to expunge his record. Good luck.
While I do practice law in NJ, this information is offered for informational purposes only. It is not intended as legal advice and you should not rely upon it to decide how to resolve this issue. No Attorney-Client relationship is intended or established by this response. You are faced with a situation where you need to consult with an experienced defense lawyer admitted to practice law in NJ, under the attorney-client privilege, before you make any decisions as to how to resolve this issue.