Praise be to Allaah.
It depends. If she is Jewish or Christian, he does not have to leave her, because it is permissible for a Muslim man to marry a Jewish or Christian woman, or to remain married to her. But if she is not Jewish or Christian, then when he becomes Muslim the marriage is annulled, because he is no longer permissible for her and she is not permissible for him. But he should give her time until the ‘iddah ends. If she becomes Muslim before the ‘iddah ends, then she is still his wife, but if she does not become Muslim, then the marriage is over and is regarded as having been annulled from the time the husband became Muslim. End quote.