My boyfriend is being sketchy?

I am fine with him going on my phone and he has a finger in my phone even. I don’t ever feel the need to go on his phone bc I trust him, but lately he has been using snapchat and every single time I look at his phone if hes on that app, he changes the app or closes it. I always ask about it and he lies and says that he was on another app. I feel betrayed lately and I want to trust him but my gut says he is doing something sketchy, I dont think he would ever cheat on me and he says so too, says im perfect “why would I ever cheat on you”. What should I do!!?