Can a person close to you sense when something is amiss?

Changes in behavior of that kind could be saying something about him more than you. I would look him in the eye next time and tell him he better not be cheating or thinking about it unless it is something you are okay with, which is rather unusual. If he is he may feel guilty and end it based on what you said. If he is and tells you he is going to end it, you may have more to worry about than if he tells you a lie. That depends. This is all conditional. You know a heckuva lot more than anyone here. Just don't be naive. Sounds like you have a commitment, but it isn't the way these days most places to expect realistically that a man will never cheat on a woman.