If services/residential program/inpatient/other are really not available, it might be good for your son to experience the consequences of his behavior---in a humane way, but if need be through the criminal justice system. Fines to pay/work off. Time in a cell, with education from social worker/police on what his behavior will result in once he is no longer a "vulnerable 13yo" or if he actually harms someone. It sounds like he has learned, so far, that his aggression has few, if any, real down sides. Sorry, it might sound harsh but he clearly, if the above is accurate, may not have an adult life if this is not dealt with.