How can I add another name to my .mac account?

There are two ways to do this
1 you can create an alias email address i.e. YourSon@mac.com however this still resolves to your dot mac mail account. You can use rules within your Mail app to sort the emails but this is a work around.
2. You can pay for a family dot mac account. $179 per family gives you 1 master account and 4 subaccounts with their own email addresses. This is a more expensive option but it provides superior functionality and inter-account privacy.
The dot mac website has all the information www.mac.com

The 3rd way is to add an e-mail ONLY account to your .mac for $10.00 which is what I have done for my son for a few years now.

To add an email only account:
Go to .mac home page, click on Account or Silhouette icon;
re-enter your password;
click on Buy More button;
select the options you wish to upgrade, which includes up to 10 email only accounts;
click Continue;
enter Credit Card info and proceed as directed through checkout process.

:)