He's really lucky to have you, it's clear how much you care about him. It's not easy to change the opinion you have on your stutter when you've been viewing it as an horrible thing for most of your life. I think he knows you don't mind it but it's gonna take some time for him to truly understand it and move on with it. Since you can't be there, sending him a message or calling him before the presentation will do. There's really no other way to help him out of this, it's a thing that he has to overcome. Showing him you are there for him is enough, i assure you he appreciates it.