My brother was assaulted by police

My brother and his friend were walking near their house at around 11ish and while walking cops stopped them and asked them to go home and after a very slight argument on why they should go home, they started walking home and after 1km my brother sat down cause he had a ACL injury and few months ago and had basically fully recovered. While sitting down the same cops came up to them from their car and started to should at my brother and his friend and then without hesitation started to hit them with a stick they at first only hit my brothers right leg( which was not the ACL INJURED LEG) and when my brother pleaded with them and told them not to hit his other leg was injured and had surgery done on it the cops straight away hit him on that leg a lot of times and hard And after the cops went my brother couldn’t even walk and had to call his one of his other friend to pick him up Please tell us what we can do We are going to the police station in the morning How do we prove everything and file a fir or a complaint