Unfortunately, you're probably screwed. Your boss is definitely an ass, but a speech impediment is something related to the job of talking to people on the phone. You might be able to get somewhere if you keep pestering HR, but even if they side with you he can just do the same thing and give some cockamamy excuse.
Sorry, I wish you the best of luck, though.
Not to derail the topic, but I too am very concerned about discrimination in the workplace. Living in the South, it's stressful knowing that, should my boss find out that I'm gay, he could legally fire me for it. Not to mention possible harassment.
I can't imagine how [censored] it must be to be LGBT down there. I'm trans, so I've certainly got the threat of discrimination hanging over me, but at least I'm in a fairly progressive area of the country. =/