The only reason you need to open ports is because your computer is behind a router. Just because it is behind a router does not mean it is any more secure than not being behind one, in which case you do not need to forward ports unless your firewall requires it (but usually you can just make an accepted programs list which means you don't need to). The reason you have a router is not for security, it is for allowing more than one computer to share the internet. If a hacker wants in, they will get in regardless of your ports forwarded. Usually the thing people worry about when they have a wireless router, is leachers (people who latch on to your connection, or even see what pages you visit).
Seriously, your stepfather should just admit that he knows nothing about computer security and should stop trying to win this argument.
