It depends if his browsing is through a (transparent) proxy, if you know it isn't, you can just head over to
http://www.privacy.net, his IP will show up.
If like me he uses a ISP which has a transparent proxy (e.g. NTL - UK), he can't get it this way since he'll get the IP of the proxy, he could try logging into his router - it should be shown somewhere, it should be shown somewhere on both Netgear and Linksys brand routers.
Failing that, he could try sshing to your box, (assuming you know your IP), once he's connected (no need to log in), just type in a console at your end:
$ netstat -n | grep 22
The output should look something like:
tcp 0 0 127.0.0.1:22 his.ip.is.here:somehighnumber ESTABLISHED
Good luck!
Steve
Edit: Or you could just to Kloth.net:
http://www.kloth.net/services/nslookup.php
Below the nslookup table is a small table with small text: "You are coming from IP address...XXX.XXX.XXX.XXX"