He is basically adding to each part of the equation to make multiples of 10 - 15-7 becomes 18-10. It is a neat trick.and should work for any number (and lots of people use similar things for mental maths).
BUT - how old is your son? A trick shouldn't really be needed for the examples you gave, by the time the child's maths is good enough to manage the trick the basic facts should be pretty anutomatic. AND does he know what he is doing and why? The details you gave of his explanations sound like he doesn't understand well enough to explain well. If he is just doing it because it is his latest neat trick and he is that kind of person fine, if he is just doing rote manipulations on basic maths not fine.
