What should a 13 year old boys blood pressure be?

Thing to consider. Besides age, height and weight are also a factor when you determine blood pressure. As a very broad guideline anything above 120/80 needs to be examined, keeping in mind that fluctuations in blood pressure are common in teens due to their increased physical activity if they are engaged in sports and also due to stressors in their everyday life. If it is continuously increased there is concern.
Depends on height! BP depends on the height and weight. If the 13 year old is of average height ( 50th percentile) the range is 108 to 133 systolic and 62 to 89 diastolic.
110/70 +/- 10. This is ideal, some variation by genetic differences.