Is your son planning on doing this project? http://www.sciencebuddies.org/mentoring/project_ideas/Sports_p002.shtml
If so, the difficulty rating level for this project is "1" - making it relatively straight forward and simple. What grade is your son in? As with most experiments, they can often be made as complex (and challenging!) as the person performing the work.
This experiment (if this is the one your son is doing) looks as though it requires a sample population to be studied and a set distance to be walked. The number of steps that the person takes to walk this distance is plotted on a graph against their height, and the process repeated for different people. (It is probably a good idea to do a few tests of each subject!)
Hopefully, this graph will produce a relatively straight line. Now, depending upon the amount of math that your son has taken, he might be able to calculate y = mx+c (the equation of a straight line). I found a really neat website which explains these concepts that may help: http://www.teacherschoice.com.au/Maths_Library/Functions/y_=_mx_+_c.htm
(I think this is probably where one of the websites linked on the project website came up with a multiplication factor of 0.413, in order to estimate an individual's height. However, without actually doing the work, I'm just guessing.)
Basically, this graph allows you to estimate an individual's height based on the number of steps they take to cover a given distance - or, to look at it another way, the length of their stride. Obviously, this is only a rough estimate and will change when the individual is walking or running! (This could even be another aspect to the project.)
Hopefully this helps a little, but please be sure to post back if you have any questions.
Best wishes and good luck!