Problem One Friend Can't Connect (Infinity 1.7.10)

[Twicebitten]

Neither of us are terribly tech savvy and we're stumped. Several of our other friends have been able to join this home-ran server, but not him. He can connect to other servers, both FTB and vanilla, but not mine.
Here are some things we've tried:

• Creating a vanilla server (he can't connect to that one either)
• Reinstalling java and the FTB launcher/mod pack
• Turning off both of our firewalls temporarily
• Creating 25565 ports in his firewall, both UDP and TCP
• Changing the port mapping in my router to accept different external ports that we tested (25570 and 80) and map them to the standard port (25565) on my server
• Changing the port mapping to 80 all the way through (passing 80 through the router and updating my server.properties to match)

Again, other people have been able to connect to my Infinity server and he has been able to connect to other servers. We've looked in other forums but can't seem to find a fix for our problem. Any and all help would be greatly appreciated. Thanks in advance.