When you get top surgery, nearly all the breast tissue and ductile tissue is removed, so he would not be able to nurse as someone else suggested. Hell, with most procedures, your nipples aren't technically nipples anymore, they're grafts, so no chance of leakage ever again. Because the actual breast tissue will have been removed, he probably wouldn't have serious swelling from the pregnancy hormones, as the hormones cause glandular tissue to swell, although it is possible that the weight gain from the pregnancy would pattern in more feminine spots, which might leave him with the appearance of small breasts. To be safe, I would advise waiting until after the pregnancy to have the surgery.
The surgeon would probably have a better idea of how much a pregnancy could effect post-op chest results, but I'd be careful having him ask that sort of question because a lot of surgeons would have reservations about an 'elective' double mastectomy on someone planning to give birth in the near future, particularly if they've not been on HRT. I don't judge either, if a FTM feels comfortable giving birth or can tolerate it to build the family they want, more power to them, but from a standpoint of the medical community where the grasp on trans concepts is often very tenuous, such acts can represent indecision and make some surgeons nervous. That might be a question for an anonymous email.
