Depends on Height
Your son's ideal weight all depends on how tall he is.
Remember also, that there is a ton of variability for what normal is at any age. Some adult men are 7 foot, some are 6 foot and some are 5 fort and none are wierd or bad. Three year olds are all different too. Have your doc show you his growth curve. If it is rising at the right speed, then you are fine...