My 5 month old head grew 1.5cm in two months (3-5 mon). i read it should grow 1cm/mon.he was above 95th, now is a little below that. is it a problem?

Check velocity of : head growth from fetal ultrasounds or at least from birth-now. If it's always grown at > 95th%ile, his development has been typical in all domains & a parent has a >95th%ile head circumference, it is of less concern than if it was at a lower %ile initially, then accelerated to >95th%ile. You need 3 measurements taken the same way by the same person to know if head growth has truly slowed down.

Growth: Everyone grows at their own rate and averages are about combining those differences. Also length is hard to measure accurately so it is more important that your baby is growing . Seems like he is growing well.