If you live in the US this is textbook discrimination and protected by the ADA. I would speak to your universities student or disabilities resource center and explain this situation. Let me be clear, this is completely out of line and intolerable. You're doing everything right, not letting your stutter stop you and not wanting special treatment and you're getting discriminated for it and it could cost you your grade. If you don't have a student or disability resource center, talk to the dean. This is majorly fucked up and that professor needs to get slapped straight.