You both will have to have an xbox live enabled Gamer Tag. Meaning you both have to have a subscription to xbox live. If he already has a gamer profile on a different xbox, you can get a memory card and transfer it over or just keep it on the memory card and it will appear on the list of profiles.
The xbox live subscription is tied to your Gamer Tag. So each will use their own account and their own Gamer Tag.
Mainly because of the way the Gamer Profiles works. For each and every 360 game, loads and loads of info is saved. If there is no specific profile and you are trying to use a Guest account, there is no place to save this info so Microsoft standardized it by having the Gamer Profiles.