No, you can't effectively sue the city or police--unless I am misunderstanding the question, the assault occured before police would be on the scene, even if they had responded instantly: therefore, you son's injuries had nothing to do with the poor police practices, and those poor practices did not cause his harm or medical bills. Therefore, the city and police are not liable for his injuries, since they did not cause them; and if you did sue for their bad behavior, all you could get would be the losses (e.g. economic losses or costs) provably caused by their poor practices, which may be minimal.