People have a right to privacy, and he has a right to privacy just as you have a right to privacy. There could be any number of reasons, other than cheating, why he wishes to keep his phone private. Sometimes pushing people in these things can lead to greater conflict.
Also, when people communicate via WhatsApp, etc, there is generally an assumption that the conversation is private. (Whether or not it involves cheating) For a third party to read it is also a violation of the other person's privacy, especially if they are talking about sensitive things like personal problems, work problems, legal problems, etc. So it is really not appropriate to read someone else's private conversations. Certainly if I talk to someone on one of these platforms, I don't expect their spouse to be reading it (regardless of their gender).
Similarly, spying is not appropriate. Just as we wouldn't like to be spied on, we should not spy on others.
Might I suggest that if you are having the idea that he is cheating or contemplating divorce, it isn't really about the phone itself, but maybe there are other things that are bringing these thoughts to mind. So maybe it is good to address the current issues between you two (rather than focusing on what if questions). If he seems distant or something else, maybe there is something in the relationship between the two of you that could be addressed, if possible.