We have the same PIN for both our phones. On the privacy point...it comes down to trust in all ways I think.
So, I would never snoop through his messages or check his email, nor would he do that to mine. It would be a violation of trust. I have, occasionally, asked to read a whastapp group discussion he's been in, or shown him part of one I'm in- things like, he's in the neighbourhood watch WhatsApp and I'm not but occasionally I like to catch up. But I ask each and every time, even though I'd happily pick up whatever phone was nearest to take a photo of our child, or google something, but I'd never look at anything 'personal' like messages without asking, and vice versa.
If he had said at the beginning, I want phones to be private, then fair enough. But - the change is the issue.
Unless you've been pushing boundaries, if you haven't changed, then it stands to reason something has changed for him, and yes I'd be worried what that was.