Most people who stutter appreciate it if you let them finish their words/sentences. just sit patiently and wait. And no need to give a big smile or overly friendly supportive look, that often increases the self consciousness. I know it's hard on you as well having to sit there while he's stuck in a bad block, but unless you see him about to break or snap or something, just wait and let him finish. dont worry about holding eye contact the whole time or anything. Just know he's having a hard time and just needs some patience, that's it. However if you see he's extremely miserable and its just getting worse and worse and it's been like 2 minutes and you're pretty damn sure you know what he's gonna say, saying the word for him in a neutral way might be easier on him than the shame of the block. Hell even if you guess wrong, it will give him a couple of seconds to regroup and try again. but like you said, it depends on the person. I have a pretty severe stutter, but i accept it and I try not to make others uncomfortable around me by stuttering as comfortably as I can. So I like it when people wait and listen. Don't have a concerned look on your face, don't be overly smiley, dont look at him like a baby deer struggling to walk. just wait and then go on like normal. You can only do so much, so dont try to do too much. Thanks for being to courteous and interested!