Ok, I still think something is wrong if his coolant is fluctuating that much(from min to the top of the tank). In my car, and the other Civics and Integras I have serviced, the variance in the coolant level is not that great from when its cold to when the engine is at operating temperature. It could be something as simple as air or even a blocked radiator or even a bad thermostat. This is only if the coolant level is varying as dramatically as described.

 

be sure to keep an eye on your engine temp gauge durring this situation that you are having. You don't want to over heat the engine & cause a problem bigger than what is present.