It’s not fair but I know from personal experience he’s used to it. Stuttering in front of people is like being dressed in a clown suit and carrying on business as usual. He knows he has a stutter and is bravely continuing his life without care of others. Instead of pitying him think about how brave he is. It should motivate you to take more chances and not care what others think. Next time you have to make a call or speak in front a group remember him and be thankful.