Head circumference

My son was born at 9lbs 11oz (4.4kg), was 21.4in (54.5cm) in height and his head circumference measured 14.17in (36cm).
At 6 months, he now weighs 21.7lbs (9.76kg), is 27.5in (72.5cm) in height, and his head circumference measures 47.5 cm. Our concern is the rapid head circumference growth.
We were told by a pediatrician (not our regular one) that rapid head growth can be an early indication of autism, or it could mean hydrocephalus. Our regular pediatrician would like to see him in 2 weeks, she believes his head is proportional to his body.
My question concerns his HC growth spurt, this seems like a serious concern. Are there any signs we should be aware of that would indicate either autism or hydrocephalus? His development seems to be on track, but the substitute pediatrician has us worried.