There was no due process because this wasn't a legal matter, it was a school issue, and he wasn't convicted of a crime. (2nd Question) Rights aren't violated because, again, there's no legal process invlolved here. We ARE given quite a bit of factual information, and there is no federal mandate to prevent bullying. Schools have a duty, just as employers do to provide a safe & secure environment. And after hearing that this has been a 3 year process, they've certainly failed to do so. I do see how it is the bully got away with it for 3 years and it takes one incident from the victim here to spark an outrage, this is where we come to an impass of information. Here in texas, we sign an agreement to go to school, in the event of bullying the school has an outlined section in the student handbook to address the issue. If your not satisfied with decisions made, you can of course call the police, or speak with the board. I seriously doubt that the proper courses of action were taken by the father to remedy this situation. I'd bet he complained informally and it never amounted to any action, although the situation is unfortunate, and I sympathise with the victim, I myself being a parent in texas have a responsibility to read the students handbook before I sign it, and I did, so in part I blame the dad for not fixing this 3 years ago.
As for legal recourse, yes he can file a civil suit, and I can see that the damages inflicted are real, and emotional as well as physical. But just as I stated, the defense will bring into question all the proper channels that the father did not follow, and suggest that it was just as much his fault.
Edit: From your own reference on wiki i quote "There are no federal laws dealing directly with school bullying;[3] however, bullying may trigger responsibilities under one or more of the federal anti-discrimination laws enforced by the United States Department of Education’s Office for Civil Rights.[4]"