My son went for his 6 month check up today & his head measured 18 3/4.... he said this was really large.... is this a cause for concern?

According to charts. from the CDC, 18.5 in/47.625 cm is >95 percentile for a 6 month old, so this is large. Has his head circumference been large all the way along at previous check-ups? A big change would be more alarming. Your doctor/pediatrician will look at his history of head growth and decide whether this is the time for further tests: ultrasound/CT/MRI. Many times macrocephaly is familial and benign.