What if you stopped breathing for three and a half minutes?

What would happen to your brain if you stopped breathing for three and a half minutes? My nephew (15) was in a bad car accident and a broken rib pierced his lung, resulting in its collapse and his lack of breathing. He also cracked his skull and had a significant amount of blood loss. He is currently in a coma, but he should be waking soon. Will his brain have any lasting effects? I am scared that he will wake with a brain disorder that will change his life and potentially impede on his future.