6 mo old boy head circ. of 46.8 cm. wt 21 lbs. and ht. is 28 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 85 % /weight 95% & head size >98% for age. By themselves, the measurements are not too unusual. However if the HC has crossed growth lines by this amount in 2 months a study of the skull contents is warranted. Ultrasound through the soft spot or a CT would help separate the problems from a simple large head.