He should lean forward on a pillow. A nurse can stand in front of the patient and hold the patient's hand. We would like to select a site that is dependent and safely away from important structures. The ideal interspace is the 7th, 8th or 9th space, midway between the posterior axillary line and midline. This site avoids possible accidental puncture of the liver, spleen, diaphragm and descending aorta.
Some experts recommend using the interspace below the upper limit of dullness.
Of course, in loculated or small effusions, you will select the site most likely to yield fluid. There are two types of pneumothoraxes which can follow a Thoracentesis. The first one is secondary to the introduction of air from the outside. This is benign and does not give rise to any symptoms. It should be left alone.