Growing up my stutter was so bad that my childhood friend would "translate" what I said to other people. I somehow improved in the stuttering area over time without any speech therapist or therapy, but it still happens when I'm stressed and the more stressed I am, the worse it gets. It will never go away and it fluctuates. I always get stuck on my name, but there are some sounds that I also hate so I will avoid saying them. Saying "stupid stuttering" would probably make me cry tbh. Even if you didn't say it to his face or around others, it's a rude thing to say. After a long stutter or a bad one, I will often mentally shame myself for it as if I would even be able to control it. Or I would hit myself if I was alone. Nothing to leave bruises but just out of frustration. The longer it takes to get a word or sentence out, the more frustrating and embarrassing it tends to be. The worst thing you can do is acknowledge it or try to finish the thought for him. The ONLY time I haven't cared if someone did this was if we're all on the same wavelength. Asking for directions, for example. Or if I'm ordering food and mentioned my order to someone I'm with and they order for me, or the waiter knowing what I want because I am a regular and always order the same thing each time.