1. Can someone explain this to me? The dialogue in my Russian book went as follows:

(А) Вы знаете ее мужа Андрея?
(Б) Да, я его эиаю.
(А) А их сына зовут Миша.

Why is the word сын declined? Is it because it's supposed to be a continuation of the person's previous sentence? Or does it have to do with using the word зовут?

2. What's the difference between человек, лицо, люди?

3. And this is probably a very dumb question. Do people's last names decline via their gender in Russia? For instance, does a family have one last name, but girls will write it with an а or я at the end, and guys won't?