Be as supportive and understanding as you can, reminding them that their injury is temporary and how important it is to give their brain extra time to heal. Ask them if they want to just rant or if they want you to get involved and offer your advice. Sometimes they may just want to state how they feel and get it off their chest, other times they may want someone to tell them that things are going to be okay and that you're going to try to find them help for when they feel how they are feeling. Try to find a neurologist for them and ask the neurologist if they work with a psychologist that has helped their brain injury patients. Call around at community centers, hospitals, youth centers, etc, to see if they hold group therapy sessions for brain trauma patients. These group sessions would really help your child to understand that there's at least one person out there that is fighting the same battles that they are. If you cant find any group sessions try to talk to your neurologist about starting one. Ask if they would talk to other patients about how they'd feel about joining in. Most likely the patients would love to hear that something like that is available to them.