As a Christian what should you do in the following situation? What if a wife found condoms in her husband's car, one used. She confronts him and asks, but he says they must be really really old because he hadn't purchased condoms since 2011? But according to research that had been done, going by the expiration date, they had to be purchased after then. Probably this year. What should the wife do? Just believe him and move on? Or what because shes having a hard time believing that story.
The evidence is that he lied. I can't think of a reason for him to lie beyond wanting to cover up whatever he is doing. Look at it this way: Either he put the condoms there or someone else did, but it is his car. Someone used one of the condoms in the car. It certainly wasn't with you. Thus, we are left with the most likely conclusion is that he is committing adultery.
Given that fact, you have to decide. The goal should be to get your husband out of his sins. The difficulty is that he isn't being honest, so that goal will remain elusive for a while. You will have to decide what you may have to do if he refuses to repent of his sins. This is something I can't decide for you.