Your attempt to help this child is noble and well-meant. If his teachers are aware of his depression (possibly as a result of abuse and neglect), then they have the tools and resources to affect safety for this kid.
If you have witnessed abuse or neglect of this child, file an anonymous report with child protective services. As much as I detest this agency, they are the ones who can take legal action to force the parents to care for their child by various means. Court-ordered family counseling, parenting classes, etc., can be mandated by the Family Court System. The best role that your family can play is to provide a safe haven. If the boy starts acting out with rages, etc., you can offer him the choice: calm down, and talk about what's bugging you, or; come back when you can control yourself. The rages and decline in grades are the only things that this kid can really control, and if he's offered a reasonable choice, then he'll feel more in control of his own decisions and actions. Having said that, the safety and well being of your family is of the utmost importance. Even the best of intentions can wreak havoc on people simply because we cannot fix everything if we, ourselves, aren't emotionally healthy and secure. If your family's emotional health is being placed in jeopardy by caring for this kid, their welfare comes first, as cold as that may sound.
As much as we all would love to help and save these neglected and abused children, it ultimately comes down to their parents or guardians. If the adults are found to be abusive or neglectful, we have to allow the agencies to use all resources available to help the kids.