Well, you may be at the top, but she sees your son's spot in the pecking order as attainable or below her own... If it is minor and the dog is not overly possessive of food and toys, it will outgrow itself.
I don't know how old your son is, but it would be good to actively reverse this perceived pecking order, perhaps have your son work on a little leash or obedience training with the dog. MUST be your son doing the training and not you. It is probably more of a game, but if she is a very alpha or very possessive dog as well, you could end up with trouble or challenges with your son around feeding time or toys if you don't.
Our son is a teenager so guess whose job it has fallen to walk the dog every day. I sometimes tend to think that our son is the untrainable problem (not the dog). In my view, getting humped on the leg every once in a while is his just punishment for not diligently walking her.
Now that I know what it really signifies, maybe I'll rub it in a bit by telling him... ;-)