This is incredibly embarrassing. After weeks of trying to resolve the issue, I posted this topic, and we one more time went through all of our possible solutions.
We started with disabling the TLS option.
Him: "I've disabled it, but it's not an in-game option."
Me: "... What? No. It is an in-game option. Go to Settings --> Options --> Lobby, and disable TLS encryption"
Him: "... Oh... I haven't tried that yet. I only disabled it in Windows' options."
Me: " "
Apologies everyone. He can now connect. All is well. We've been trying to play this game together for 4 weeks. Finally, it's fixed.