@Comedycook
I can see his point of view: if the worst happens he doesn’t want to spend the last time before it in work
How on earth can you understand this point of view? It's absolute nonsense. Good job most people don't think like this or no shops would be open, buses running or doctors working! Just in case we're nuked and don't want to be at work when it happens 
I didn't say I agreed with him keeping the kids off school, just that I could understand his POV. With his background, he's probably been following the news more closely than most, and probably has a more informed opinion on what the worst case scenario could look like
Like most people, the concept of war is horrible to me, but I'm lucky enough to not know what the reality of war is like. It's easier for me to carry on with my day-to-day life whilst we're waiting for the world to potentially end, because I can't picture what it would look like if the world did. He's an ex-marine. He's been trained to know what the worst possible outcome looks like, no wonder he's struggling.
As I've said before, I don't think the OP is going to persuade him to change his mind, and she needs support from either one of his military mates, or if she doesn't have any contact details for them, from a military service/charity. This is a man who genuinely believes he is doing his best for his family based on the intelligence has access to - I can't see him listening to anyone right now who hasn't gone through a similar experience to him.
He's not abusive, he's not necessarily having a mental health crisis -
he just has a different POV, and that POV needs tempering by someone with a similar military background who can talk him down to a more balanced compromise, e.g. the kids going to school, but coming straight home for quality family time.