Well, that was police brutality, regardless of whether he was 15 years old or an adult. Considering there were a lot of them on the scene, my guess is that he was hiding, with all the cops floating around looking for him, he found a possible break, ran for it, he got spotted, and they stopped him. Yes, the cops went way overboard. It would only have taken one cop to cuff him and another to keep him at gunpoint. It's possible he was mouthing off when they got him down, but that still doesn't justify kicking him.
The person could always sue in civil court even if the officers involved got acquitted. I don't know if that happened.