The games are obviously not played in real time, being turn based and all, but each round is defined as an in-game time unit.

Basically, you have 6 seconds (basically quickly once it's your turn in a combat round) to declare what it is you're going to do. The idea is to try to simulate in the moment stress and excitement. Once a player declares his intent, then he follows through.

As the DM, you'll just have to state a game approach. How long does a player have to choose what he'll do? What will be the standard result if the player doesn't choose quickly (the PC will spend that round fighting defensively, etc). Spur of the moment changes based on unexpected results are always possible, but they do have to be spur of the moment.

You'll just have to talk with your player. Tell him that he is welcome to have a tactics discussion with the group, prior to combat, or before starting a task, but actions inside a combat or encounter have to flow quickly in order to have the proper flow and feel.