Maybe he has never cheated on you physically. But, it sounds like he is cheating on you emotionally.
He is either not happy or fulfilled with the relationship and is using "sexting" as an escape from reality that enables him to live in his own fantacy world or he has some other underlying issues.
If I were you I would tell him to stop and to get help. If he is not willing I would leave him. If he continues "sexting" chances are you will not trust him, may start to feel inadequate, etc. This is not the way to live your life.