Things like that are typically triggered by something: the environment or an event. Are you an anxious person? How do you deal w/stress? As his most direct role model, does he see you relaxed? How often? Would you say that other influential people in his life are low key and calm?
The other thing to consider is: when this behaivor started. Was there an event that may have triggered this behavior? Did you and your ex ever argue in front of him? Did he suffer any trauma that my have jumpstarted an anxious nature.
If it comes down to an event, talk about the event with him or more importantly . . . listen to what he has to say about it. Don’t treat him like a kid. Be open, honest, and reassuring. Don’t be afraid to get him counseling if he needs it.
If he’s surrounded by people who are keyed up, always jittery, running late, worrying about things, full of stress, get those people out of his life. If it’s you, calm down. Become the stress-free mom your son needs. His emotional health and happiness depend on it.
It’s important to get to the root of the behavior. Everything else is window dressing. Start at the cause and diffuse from there.
Other things you could try: create a calm, reassuring atmosphere at home. Teach him to meditate, play soothing music, turn down the lights, turn off the tv, cell, internet, video games. Eliminate all caffeinated beverages. Reduce sugar in his diet. Enroll him in a martial art. Talk with your son about what frightens him. Be there for him . . . every single time.
Good luck!