what is noc (sp?)
For clarity; he is using YOUR PC and keying in HIS username into your DSL modem with no other changes?
Are you getting login/authentication failure messages? With his account logged in, can you go to bellsouth.net and check your webmail account?
It sounds like it could be a problem with your userid/password; their B-RAS won't hand out an IP without a good authentication and you can't surf without a good IP.
A BellSouth tech should have a way to escalate into their NOC - demand this if it is not offered.