@thearst,
No, you are not paranoid. He is obviously hidding something.
I had the experience with my wife. She was acting strange, i tried talking with her, and consantly denied cheating. She even get mad against me for even daring to accuse her of cheating. Sometimes i even feel as a bad person. She was quite a good actress.
Then she acted as a good wife a couple of days and then resume her strange activities. This continued for months. With me trying to talk, only making her angry. I finally get tired and did a small search and discovered her affair.
I is very difficult to move on. I now this by first hand. I hurts like hell. As i see this, there are two options: do nothing and learn to be happy with the relation as it is, or go on and investigate.
If you decide to stay the way things are there is nothing more to do then. Many people i know had done this.
If you decide to investigate yo have to be sure what you will do if you discover something bad. Will you be prepared?