Hie lie is probably going to be that it's not his phone. He may even pretend he's keeping it 'for a friend' or 'found it and is as interested in the contents as you. Remember, IT HAS HIS MUM's PHONE NUMBER AS PIN. It was in HIS BAG. It has texts going back to JANUARY. It has ONE NUMBER.
Keep reminding yourself of this. Make note of any names used in the texts or details of place and time. But mostly, confront him with confidence and simplicity. Keeping the phone I'm not entirely sure what you would be waiting for. A text to it? It seems unnecessary.
You should act now. Put the phone on the table once the baby is asleep, look straight at him and say, 'Tell me about this. And please don't lie, this is your one chance."
Let him fill the silences. He will pick up on your uncertainty and sadness and fear. Keep repeating the facts in your head, and eventually and when appropriate, to him. For example when he says, 'It's not mine' look at him, you'll know if he's lying, and say 'Yes it is' because you know it is (pin). When he says, 'What do you mean?' as a way of buying time, which he may well do, just say, 'You know exactly what I mean. Tell me the truth." Over and over.