No a man cannot say that. He will typically say, "we are pregnant" meaning he and his wife. The word conceiving is really never used in this context. Conception is the moment a woman becomes pregnant so conceiving would be the act of getting pregnant. We just say that a woman is pregnant or a woman is expecting.

It's not natural for a man. say that except perhaps as a joke, but he might say "we're pregnant" when referring to himself and the mother of the baby. You can say a woman has conceived, but I to say she is conceiving is very natural and suggests she's currently engaged in the activity that will make her pregnant.

In his essay "Shame", Dick Gregory recounted his childhood. He used the expression as a metaphor, saying he was "pregnant with poverty."