I essence, you are correct.
But it depends where and how your son is connected to the network.
Over the Internet, you can only access another computer that has a public address. In other words, one that is directly connected to the Internet.
If your son is connected via a router (either wired or wireless), the router would have to be configured his end, to port forward to your sons machine.
An alternative may be click here Apparently there is a Debian package that should work on Ubuntu. So it may be worth a look. But I haven't personally tried it.