Hi mother in law

I am actually glad to hear that
a) the watch is in place
b) your GS knows about it
c) Your GS is 10 and is old enough to pick up a phone and dial 911 if needed.
of course, we wish that he never found out or needed the watch at all, but since BPDmom let the cat out of the bag, you are right that Dad needs to deal with it and help him process what it all means.
I hope I can help a little with explaining and normalizing this for your GS10.
I am a Non (BPD exH) married to a NonH with BPDexW who lives in another state. We are residential parents. I am raising 6 kids, 3 step sons and 2DD from previous marriage and 1 bio toddler son with my H. Ages 2 - 16.
My exBPDH is from Mexico. There was a time during the early separation weeks that he made statements that led me to believe he might take my DD then 5 and 8 to Mexico. DD are now 13 and 16.
When he intuited that this was a concern when the OOP (order of Protection) was amended to allow for visitation he told me,
" I would never take them to Mexico. They love you. They would not listen to me."
I felt better, but not 100%.
As they got older and his parents became ill and his family kept asking to see their GDs then he started talking about taking them there for a "vacation". It was his right! I became increasingly concerned.
Around this time, he became verbally then physically abusive, was acting erratically during visitation (more here for another day of stories)
At any rate, I filed a motion to suspend his visitation due to many things including all of the above, non payment of support, as well as his threats to take them to mexico.
My DD were 9 and 12, around the age of your GS. They DID NOT WANT TO GO TO MEXICO. The GAL agreed it was unsafe due to his mental state and it was also fairly dangerous to travel there due to the narco trafficing violence.
In the end, he fled the state, refused counseling, and my DD are happy as he mostly leaves them alone.
Problem is we still have joint custody on paper and they need his signature to get a passport until age 16. I do not want to ask him for a signature for DD13 as it would trigger him. So we are not travelling out of the country any time soon... .
So that is my story
As for your situation, I think I am seeing a number of ways to approach this
You might want to normalize it by saying,
Well GS10 when parents get divorced, sometimes the kids are put on airport watch (only use this term if you have to if he knows it already) because the judge thinks its safer because mom and dad were arguing too much.
You know Mom gets really mad sometimes, she sometimes says things she doesn't mean, but this was not done to punish you or her. This is a decision made by a judge.
When you are older you can absolutely go to see your family there.
Right now it is just not safe.
As for BPDbioMom, well, the legal system has set a limit. She will go ballistic and continue to, then eventually she will die down or not.
You can not control her reactions.
I think being as honest as you can and as calm as you can is best.
Also, I highly recommend therapy for GS10. Especially if Dad and GS10 can talk with a therapist about all these confusing things going on.
I hope this helps.
There are many resources here.
A great book to read is
"I don't have to make everything all better" by Lundbergs. I recommend it for you as well as your son.