I would try rewarding him every time he doesn`t spook. You can set up situations where you know he`s going to spook (for example, having someone walk into the room or slam a door). Before the `spook` even happens, take out a millet spray and let him get really into it, and then when the scary event happens he might stay put. Purposefully cause spooky events like this several times, always making sure he`s distracted by food or a toy, and praise him when he fails to spook.
Then you can try doing things like presenting the millet/toy just an instant before the spooky event, then at the same time as the spooky event, and eventually... after the spooky event. At this point you`re rewarding him for not spooking. If he knows he`ll get a reward (like food, toys, or praise) if he doesn`t spook, he might be able to learn to stay put.