If you go to speech therapy you'll learn that you do have some measure of control over with it. I don't know why you lied to your boss either. Personally, I have a severe stutter. However, when I slow down my speech and stretch my words I can speak fluently and I do have much greater control even if the price is that your speech sounds a little less natural. I'd encourage you to be open about this to your boss. His responsibility is to the company and I would also be concerned if one of my employees who was pitching presentations was going through severe blocks. No matter what he says it's ultimately your discomfort and you're probably the one who wants to stop doing that more than anyone. I know because I experience hard blocks. I wouldn't wish it on anyone.