The guy seems to target them in social situations so it might be hard to ignore. I don't think going to hr is a good solution either (ik you didn't mention it, I'm continuing with my general opinion) since what he's doing might seem normal to normal people, it's only an issue when we consider op's speech impediment and hr might not understand that tbh, they'd probably just reduce it to social anxiety and that might make op look bad in a professional setting where people are supposed to be self assured. I really don't know what's the best course of action here, except from enduring it and spending a lot of time studying techniques for core symptoms and others for social anxiety, but if it becomes too hard, it might be useful to have a psychologist and/or speech therapist's opinion on how to mange work, they could support op with a letter to present to hr, even simply acknowledging the difference in public can bring a lot of relief