Can a man say "boruch hamvdil bein kodesh l’chol" or must he say maariv for Shabbos to be over? If a man has not yet davened maaiv is there any way that he can do melachos?

Shalom, All Jews - both male and female - must keep the laws of Shabbat until the time when three (medium) stars appear in the sky (which is printed on all Jewish calenders for each location, and can be found on the home page of the Yeshiva.org.il website). After that it is still forbidden to do any melachah (forbidden Shabbat labour) until they have said (or heard said for them) a declaration of blessing Hashem for distinguishing between the holy time and profane time. This declaration is said during the evening amidah, during the havdalah service, over the cup of wine, or may even be recited by itself - "baruch ha'mavdil bein kodesh le'chol". So - to answer your question: A person may recite "baruch ha'mavdil ..." after the time Shabbat is over and then do weekday work. This can be done even before praying the evening service. They still must recite this again in the evening prayer, and again over a cup of wine during havdalah. Blessings.