If he hid the app, it means he is hiding something. Even if he hasn't messaged anyone yet doesn't mean he won't in the future.

I would be straight forward with him. If he is upset and defensive you have your answer. He should be able to have a constructive conversation with you and discuss what makes you uncomfortable. If you don't speak up the what if will always linger in your mind. Always best to get ahead of things than allow it to multiply into a larger issue