How much should a 3 1/2 year son weigh?

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...
It Depends. This depends on your son's height. Weight is referenced to height. A body mass index (bmi) is often helpful to determine appropriate weight to height.
Use growth chart. Ideal weight depends on height and body frame. You can enter your child's birthdate, height and weight on blubberbuster.Com and it will give your child's weight percentile and weight relative to height. In any case, a typical 3-1/2 year old should weigh between 30 and 40 lbs.