Ugh? that sucks.
I really don't blame him though. It's a real bummer to think that you can trust people with something. I've had to sign a bunch of NDA's and secrecy contracts working in this (film) industry, and it's never been an issue for me to keep my mouth shut. But it happens all the time. I can give you an example:
(Don't worry, the project is done, and the NDA is expired)
I was called on to play a soldier in August: Osage County. (I don't know if it's any good or not, as I haven't seen the film, nor do I have any idea of I'm in the final cut). The casting director called me the week before I was to be fitted (unusual because this type of communication typically happens over email) and tells me to be ready to go at a moment's notice. A few days later, he calls again and gives me a time and place. The time was later that day. A few days later I get another phone call with another time and and place, which I was to be at the next morning (the call was around midnight). This practice was done with every person on the set, and the location was still leaked. It doesn't bother me to keep secrets. I'm former military, so it's no big deal for me to operate in a world where you can say nothing about what you're actually doing. My family jokes that I actually work for the CIA.
Needless to say, some people just can't keep their mouths shut. And they really need to.
Especially if we're going to lose films like Hateful Eight. I was looking forward to that one.