What's the point? If he is, he could just lie to you...
I think you let it be and watch his actions, and when the time is right I would ask him about openness in the relationship and ask him if he minded showing you his phone.
And besides the point, the phone itself is a huge tell. Is he taking his phone everywhere, is the phone locked? Does he ever leave the phone out of his sight?
You will know if something is up, for now just STFU and go with the program.
If you start with the jealousy thing, you know what will hit the fan...
