I've never spoken to a person who "pretended" to be Dominant or was able to act that way unless it was in them. I do it because I love it, because it's in me, its who I am and I often do it unintentionally because it is so deeply embedded in the way I think and see the world. I don't understand Dominatrixes (sp?), because I'd never be able to charge money for what I do...if I wasn't committed to my boy I'd constantly be giving it away for free. : )
But the distinction between Dominance and topping is an important one. A person most certainly can learn to top, and can probably do a fairly good job of it based only on the knowledge that their partner enjoys it; but this would only work if they then got something that they found fulfilling, too. One of the best pieces of sexual advice I ever got was "it's perfectly fine to do things just for the sake of your partner, so long as you feel they are fulfilling your needs also." In short, he can learn how to spank you the way you like, and say the things you want to hear, but you'll probably have to ask for it most of the time, and then your following activity should be something he enjoys.