Does your boss already know your stutter? I disclosed my stutter to my line manager so she wouldn't misinterpret this as me being nervous or scared all the time when it's just a thing about me. She didn't mind and I've never felt discriminated for this so far. For all you know, your boss may not even care about your stutter either and will see you as much more than a stutterer. Overthinking can be very emotionally draining when at the end of the day, it could just be all in your head. Take deep breaths, maintain good eye contact and have appropriate facial expressions. If you block up, you block up. Don't beat yourself up. Don't even worry about it. At some point you tend to realise there's no value in remaining fixated on fumbling and blocking up on words when the other person shouldn't obsess or make such a huge deal over this and if they do, that's their problem alone.