He can't let the stutter hold him back from opportunities forever. It's best he discloses the stutter to the manager so they're aware and don't assume it's to do with any form of incompetency when I'm sure he can get the job done at the end of the day and that's all that matters. Most people should be fairly decent and won't make a big deal out of some stumbles here and there in talking. I feel if the more he practices talking to people then the easier it will become to be confident, stutter or not!