Do you have any indications that he is cheating on you? What are the things that lead you to believe this? Is it just a hunch? Does he go out after work and stay out late? Is he gone on the weekends? Does he ignore your calls and texts? Gather together what you have as evidence and show him. If you don't really know and you are just suspicious or jealous, maybe you need to examine the root of your own jealousy.