Go to ed store > login > go to "Licensing" in the personal section > enter your FC3 key > type the name of your son's account > click bind
I hope this helps !
It should work if he logs into multiplayer with your account, or whichever account it is bound to on the ED store (you can bind a product to a different account if you wish). I bought my DCS products under a couple of different accounts on the ED store so I had to rebind them all to one and it works great now.
I love your MiG videos man, very professional, informative and straight to the point. However, it goes without saying that I can't wait to fly this plane myself :D