It’s easy to set up another account so each of you can log into your own account, and if he changes the default browser to Firefox in his account, it won’t change your browser.
Look into the Account Preference Pane (it’s on the System line of icons, fourth from the top) and you can add a new account using the little + sign at the bottom of the account list. There is probably only one account right now: the administrator account under your name. You may need to click the lock icon and enter your administrative password to add the new account.
So what you need to do is add another account for him. But you do not want to make that account an administrative account; that means will be able to login to his account and do things, but not install programs and not change anything with the accounts themselves. You want to keep that administrative power to yourself.
This help page at Apple explains more about user accounts: Mac OS X 10.6 Help: What is a user account? And here is a help page on setting up a new account that also explains more about the differences between administrative accounts and other accounts: Mac OS X 10.6 Help: Setting up a user account. You will also want to read Mac OS X 10.6 Help: Quickly switching between accounts to see how to switch between accounts without restarting the iMac.
Depending on his age, you may also want to control his use of the Internet and the sites he visits. See this KB article to see how to use Parental Controls under OS X: Mac OS X v10.5, 10.6: About the Parental Controls Internet content filter.