Each sub-account of a MobileMe Family Pack should have been individually migrated to iCloud before August 1st last. Assuming that you didn't do this, you should be able to reactivate the address (though not retrieve its previous contents) by signing in with it in System Preferences>iCloud and enabling Mail - your son should do this on his own Mac or his own user account if he is using your Mac. It's not clear whether he will be able to reactivate the @me.com version or only get an @icloud.com version. (It would be useful to know, when he tries it.)
If he did migrate the account to iCloud then signing in in the same way should retrieve the contents and the @me.com address should work.
Alternatively he can just create a new Apple ID (for which he will need a new non-Apple email address) if he doesn't already have one, and sign in with that, enabling Mail and choosing a new @icloud.com address.
You should note that in none of these cases does his account have anything to do with yours (there is no need for a Family Pack equivalent in iCloud as it's free at basic level).
You could add an email alias to your account, but it's not a good idea as it will mix his emails up with yours and he won't be able to detach it later.