Worried about my fiance

We've been engaged for 6months now, he told me recently that when we first started dating that not only his ex lived with him but that he slept with her, and it only happed once. Now I'm worried that he might do it again by the way he's talking about his past and being able to sleep around with whoever. Specially since I haven't been able to get pregnant. I just don't know what to do anymore