I can understand that it starts to get somewhat annoying if people block for a very long duration of time, but never finish his sentences for him. It doesn't help him, and probably even lowers his self esteem even more. Me myself I'm a very mild stutterer, so my blocks usually don't last longer than max. 1 second, but from the past (when my stutter was more severe), I hated the few times when people said my words for me. Also, blocking for such a long period can be prevented. You might want to tell him (on a laid back moment) that he shouldn't keep using force if the sound doesn't come out. Just tell him to relax his articulatory muscles and 'let go' out of sound, and then carrying on with the rest of the word. Just like instead of b....(keeps pushing).....lock, b.(relax muscles and say the rest of the word).lock. Stuttering itself can't be prevented, but we have much more control on our secondary symtoms (e.g. the blocks, repetitions, prolongations, hand/face/leg jerks, etc.) than we realize. Own your secondary symptoms and you'll be just as fluent as everyone else, just sometimes with a sound that you 'skip' over. Ever heard a child stutter? They do so much more smoothly than older PWS.