Unable to connect on 2nd computer on network

I created a server so my son and I can talk while playing games together, and not yell across the house, but he is unable to connect to the server on his computer. My modem has nat loopback as the game we play is run on our network. I do not know if anyone can connect outside the network and I have no way to check that, and this was created solely for him and I to talk to each other and I don’t want others coming on it anyway. So it is password protected. And yes the password is typed correctly on his computer. The server is run on my computer on windows 10. I am able to connect no problem on my computer. How do I fix this?

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