Grogger: It should be very easy to play multiplayer with games that work like Terraria, the most complex part might be setting up port forwarding on a router if you want someone over the internet to connect to your PC/server.
If you gave some details on how you and your son are trying to play, ie your network setup (over the internet? Over a LAN, router etc?), I'm sure someone would reply giving you some quick instructions on getting it to work.
txsizzler: We play through a wireless router (Netgear 7000). Why do we have to do a complicated process to play the game multiplayer? I know this has been brought up a ton in the past, but now this is more of a sticking point with me, as Steam has an easy way to play with your people on your friends list. Yet, the GOG version requires Port forwarding and/or a separate application to do basically the same thing. Very annoying.
Without getting into a GOG vs Steam thing, or how much a developer decides to support one or the other (which seems to be the real issue here since Galaxy), this should help you get going with some multiplayer over a LAN.
Open the game and choose to host/create a multiplayer game (this will be called the Host/Server computer), entering any details it requests. Then on any other computer on the network that wants to join, open the game and look for a multiplayer/join game option.
Here you will have to enter the IP of the computer hosing the game and a port (leave the port on a default value if you haven't changed it when creating the host). It may also ask for a password, which you may have set on the host. From there it should attempt to connect and join the game instance on the host.
Problems you may encounter is different game versions not working well or at all with others.
Firewalls blocking the program from connecting, on either the host or client side.
Not knowing what the Host's IP address is.
If you don't know the IP address for the Host on the LAN do the following. On the host PC, Run (WinKey+R) "CMD". This will open a DOS like text window. In that type "ipconfig" and it will display some text. Look for a line that says "IPv4 Address...." at the end of that line will be a number (such as "10.0.0.8" or "196.0.0.9" as examples) which will be the LAN address for the host, that is the IP number you enter on the client to join the game on the host.
The only difference between doing this over a LAN and the internet is, there you connect to the Host's "internet" IP not the LAN IP and you will have to tell the router to forward the port chosen when creating the host to the host's LAN IP (port forwarding is only needed on the host side, ie if you're hosting over the internet). That's also very easy but exactly how to do it depends on the router/firmware in question. You don't need anything else like programs or websites etc for either LAN or internet games.
So it's not really complicated, basically all you need to know is the IP address of the host computer (and password, port if changed), so the client can try connect to that IP where the game is being hosted.