You can do virtual hosting or URL re-directing/masking. The first is something you can do, the second is something you do at the domain register.
By the sounds of your question, you have a web hosting location. Do you have control over the Apache config and/or DNS of your domain? If so, you can set up the "Virtual Domain" in the Apache config so that when a request is made for his site(on port 80) or another port (if you want to get fancy), it routes it to another folder on your server rather than the default. That way you can host multiple sites off one IP. Then once you got the Virtual Domain settings up and running, just update DNS with the server IP and you are golden.
If you do not have control over the apache config, you might be able to do it a dirty meathod with Domain Masking. You can set up a folder for him on your web host. Then, on the domain side, you can have it "mask" the re-direct of traffic to the location. It can even keep the domain name in the location field while you surf. This is effective for those that have crappy hosting from a free site.
Hope this stuff made sense