It all depends on you. Would you be comfortable dating this person if he sees you as a female? If so, then there's no rush. If not, in order to be on the safe side and not get hurt/get really dysphoric about this, I'd say let him know you're a guy. Even pre-op. Your parts may not match the general view of what a guy is yet, but your mind is. And yes, this may raise questions, but it could be an opportunity to educate him a little. Who knows, he could be in to you being just you, regardless of what gender label would be attached.