If you do a Google search you can probably find a few dozen different sites that offer a few dozen different tricks for improving stride. My personal view is that your body is going to run how it wants so there's only such much you can do. I think the best thing you can do to help your son is to get him running more and running faster. Consistent training will help force his body to figure out on its own what needs to be done to be more efficient. The faster running also forces the issue. Have him do some short interval training a couple of times a week (some 200 and 400 meter repeats at 70-80% effort). Core training can also help improve form. Keep in mind that very few runners even at the elite level have perfect form. If your son likes running and he does it enough he'll figure it out.