I'm assuming that the systolic is 100 and the diastolic is 73, as the systolic cannot possibly be higher than the diastolic. Now 100/73 is not low blood pressure, as new guidelines from the AHA state that optimal blood pressure should be BELOW 120/80. A blood pressure of 120/80 and higher is now considered prehypertension
. There is nothing to be concerned about unless he feels dizzy or lightheaded.