6 mo old boy head circ. of 46.8 cm. wt 21 lbs. and ht. is 22 in. hc increased 2 standard deviations since 4 month check. development normal. concern?

Yes: If the measurements you listed are accurate, you have a kid whose length is far below the 3rd percentile for a 3 mo & a head circumference at the 98% for a 9mo. This may be fine for various forms of little people but the measurements don't add up to anything expected. Rapidly increasing head size deserves a study of what is inside. Ultrasound may work at this age.