What does it mean when a man dreams his pregnant?

Nothing, dreams have no inherent meaning but are simply random thoughts.

As this category is "Dreams and Dream Interpretation," here is an interpretation of the dream:

The pregnancy in this dream is a metaphor for something the man is "developing," or 'gestating" in the sense of an idea or project. This project is as important to him as if it were his own child, as in the common expression, "that [antique car] is his baby."