It is absolutely unbelievable that your boss continues to mirror your stutter after you have made it clear that its not something you can control and makes you uncomfortable. The fact that you said "the most recent time" and that your interactions with him are now a "trigger" for the stutter means you have been nice for way too long!!! What he has done and is doing is a form of disability related harassment and even if you don't feel harassed- what he is doing is wrong. I have read a few comments about maybe he doesn't realize he's doing it and I find that a little hard to believe (though not impossible)... do you feel like he is doing it to be funny or do you feel like there is a chance he is unaware of the action? If you think he is doing it to be funny or just because he's an insensitive jerk then it's absolutely time for you to contact your HR department and make them aware of the situation. Make sure that you include that this is isn't the first time he's done it- that you have tried on numerous occasions to get him to stop and he apologizes but then does it again. If he is mirroring you intentionally then you reporting him will probably help other people as well- people that are jerks are usually jerks to anyone they see as inferior to themselves.