It means he is ready to do something at any time soon. Maybe he will react to any provocation. A coiled spring will expand quickly when you let go of it.

I agree with GuideDogSaint. It means he is ready to do something at any moment and without hesitation. I think it is usually used in the sense of someone who gets mad easily though. For instance, if someone takes offense at little jokes you make, you could say that person is like a coiled spring because they will be quick to get pissed off and tell you off. They react before thinking. Hehehe :)

in england, e.g. He's broke but gets paid next Friday and then he'll go to the nightclub, he's like a coiled spring right now.