How do I convince my husband that I am not cheating on him?

Over the last month or so, my husband has been acting crazy. As soon as he gets home from work, he takes my phone and starts investigating. I'm accused of deleting calls and texts. He even gets on our wireless account, looks at my phone activity, writes down numbers he doesn't know, and calls them. We've been married for 6yrs. And have 3 children. Him and the kids are my whole world. He's never been the least bit jealous or controlling, so this has truly thrown me for a loop. He's acting like a full fledged psycho! I've always been faithful, and never do anything to make him think differently. I've tried telling him how much it hurts when he does this to me but he acts like he really doesn't care. How do I convince him otherwise? What do I do?