Our neutered alpha female terrier humps our son but never me. Thanks for finally explaining why. Other than that, she has a tendency to go for the neighbors' kneecaps, and pick fights with neighborhood dogs of all genders and sizes...
(Any application to recent events at the DNC convention are purely conjectural at this point in time... ;-)
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.