Hey all, thought I'd chime in since I have some expertise in the area. I'm actually launching a membership site aimed at improving public speaking and presentation skills in the next week or so (there's some sample content already up on the site at
www.SuccessfulCommunication.com).
Couple of things:
1. I wouldn't hire a voice coach or vocal coach as the mumbling and diction problems are more likely to be a result of his fear of public speaking (causing him to speak too quickly and slur his words) as opposed to a poor speaking voice.
2. The way to confront him about the problem is to videotape a few people in your office giving presentations (don't just isolate him) and have a critique session afterwards. Not only will this isolate a few areas for improvement, but it will also likely comfort him by seeing that he isn't as terrible a speaker as he thought he was. Even if he is atrocious, people universally think they are worse speakers than they actually are.
Drill Suggestion:
The drill I use with my clients to eliminate mumbling is to bite down on a pencil (not the tip or the eraser but the midsection) and carry on a conversation with me. If they want me to understand a word that they are seeing, they quickly realize that they have to over-enunciate which trains their lips in a particular way so that they avoid mumbling in the future. It's a drill that is astonishingly effective, but it's also lighthearted so you can suggest he try it without feeling awkward.
Let me know if you have any other questions and I'd be glad to help.