DC is not always the easiest thing to do. There can be many factors. But your situation is a bit odd. It's really hard to know or should I say guess when you haven't supplied much in the way of details of your set ups. Other members would also be at a without some info. Anyway I'm off line till later.

Details like:
1. OS version? (eg: Windows 98)
2. What firewalls you have?
3. How much ram?
4. Hard Disk space available?
5. Connection type? (eg: dialup, broadband, cable, etc.)
6. Modem & router brand name & model numbers?
7. Where are you trying to connect from (home, school, work, etc.)?
8. What version of Limewire (LW) & Java do you have? (Go to LW's menu Help>About LW...) Will show both LW & Java version.
9. Who is isp provider?
And any other details you think might be relevant (or not. lol)

Oh a late thought ... you could create a Magnet link for one of your library files & email it to him. Right-click the icon (under the '?' column on the far left) of any of your library files & select Magnet details for file. Then copy the info in the Magnet box. Send that. He can type it into his browser address bar. Then he 'should' start downlding your file. Once he is downlding from you, he can browse you. He can then try DC after that.