Put the bike on a box stand. Measure distance from the saddle bolt to the rear axle nut. Call this A. Then take the bike off the box stand, bounce it up and down a few times, now measure from the same two points, this is B. Now get your son in all his riding gear on the bike, bounce up and down a couple of times and measure from the two points again, this is C. Obviously somebody will have to hold the bike upright when your son is on it because he can't have his feet on the ground. Take B from A to get the static sag, and take C from A to get the race sag. Unfortunately I don't know what it should be for a KX...