I personally think that the moving back in was a very bad idea for all concerned.
The anxiety may be a coincidence but seems likely related to relationships and ?loss.
You could suggest taking the son to see a psychiatrist and/ or psychotherapist.
Of course I understand your shock about what happened. I can think of a number of possible explanations for what happened, but my guess is that figuring out what happened would be pretty difficult even if I could meet with the son. That is a pretty complicated time in anyone's life, and especially given the divorce.
More to the point, perhaps, since diagnosing the son remotely is not going to be possible, I wonder what I might advise the two of you. I don't know what the situation is with regard to your boyfriend's relationship, I do know that sometimes families stay together until the kids graduate from high school... but I don't have any sense of whether that is a good idea or a bad one.
I have a hunch that moving back was not necessarily the right response to this crisis. After all, if your boyfriend was planning on getting a divorce it doesn't seem likely that he and his wife are likely to suddenly develop a good relationship. It might make sense to think instead about ways that they can be more effective co-parents.
Just a thought.