I'm surprised you managed to delete this ntuser.dat file as I thought it was protected.

Is his email the only problem your son is experiencing? ntuser.dat holds other data as well, like the registry keys for application installs for that user, so I would have expected other difficulties.

When you say your son cannot access his email, what does that mean? What email software? And what happens when he loads up, (opens), that email software in his XP user account?

Things you can try;

1] Have you tried System Restore back to a point/date before you deleted this file?

2] Try re-installing the email software and creating his email account again.

3] If all else fails, create a new User Account for your son, copy across all his personal files, (music, photos, etc), into the new account, and install the email software for him.