If you can get the last part of their IP, and you know what ISP they use, simply find what the netblock for that isp is, and you have his whole IP address. A netblock is a block of IPs an isp is given to use, eg 203.134.32.0 - 203.134.63.255 is iPrimus (Australian isp)'s block of Ip's. These ip's are designated for their use only.

So, if you know what isp he uses, find out their netblock and you have his whole ip, as the netblocks always (i *think* always) have the same two first numbers.

I have a good whois tool called IPlookup. I can't remember where i got it but i think it was from sysinternals.com (i think). Any will do though. Or you can go to whois.apnic.net, whois.crsnic.net etc....there are around 20 internet registration sites with whois servers, and tools will just use these for you, rather than you have to type out everything from telnet or your browser.