What's the nature of their chats?

I wouldn't go so far as to use an app, especially if you're not overly tech-savvy. If you can get a hold of his phone, just use yours to take photos of the conversations. I'd do this before confronting him, though, otherwise he'll delete evidence and change tactics, and you won't know where to get your evidence when that happens.

Ask him about it. If he denies it & you don't believe him leave him anyway because you don't trust him. Who wants to live like that?

Try having a conversation. Tell him you've seen the messages. If he's going to lie and dent it to your face there is something very wrong.

Why don’t you just confront him? Take screen shots of his conversation and send them to yourself if you need proof.

Why do u need proof juat leave him if he is cheating.