J.B.
I live in the same area as you so I can relate to what you're talking about. For those outside of Massachusetts, we have been hit with an unusually high number of threats being made to schools in the eastern part of the state and these threats have of course been very disruptive to schools and students. On Tuesday, 15 schools received threats and last Friday, there were 7. Some schools have cancelled classes or dismissed kids early, others evacuated for a couple of hours before letting kids back in and other determined that there was no credible threat and carried on as normal.
I think for your son, you need to help him see this for what it most likely is, which is an annoying and disruptive prank that gets attention from media and law enforcement. The FBI is involved and will be looking for the source of these threats. We let them do their jobs and move on with our lives. Unfortunately, although we all probably experienced bomb threats in school (we had about 1 a year when I was in high school and we would have to stand outside for an hour or so until given the all-clear to go back in), that was before people were actually shooting up schools.
This info probably isn't helpful to share with him, but in my mind, the real threats are ones that don't announce themselves ahead of time.