Not yet. Children grow at different rates. Your child's pediatrician will follow your son's growth and if it remains consistent, this is good. If your child slows in growth considerably over a certain time frame, the pediatrician may want to look into reasons for this change. You can track your son's progress and watch for problems on a chart, but go to all check up visits. http://www.medcalc.com/growth/.
Follow trends. Unless an infant is measured with an infantometer, getting accurate length measurements isn't really possible, particularly with the paper and pencil method. The growth chart is a very nice way to follow trends in length, weight, and head circumference for infants and toddlers. A pediatrician who plots out the growth points should recognize a problem if there is one.
Maybe. The average child born at 20 inches would grow about 3.5 inches in 3 months. Height measurements done in hospitals tend to be inaccurate since the baby's head may be a little elongated from coming through the birth canal. No need for panic at this point, but your physician should follow this closely and be sure to ask to have your child measured every time he is in the office.