If he is like me my stutter gets worse when I am anxious and I have fairly severe but mostly manageable social anxiety. In my experience he should just make it known he has a slight speech impediment and it’s worse when he is nervous at the beginning of the speech and that takes a lot of the anxiety and pressure away. Also if I am reading from something I am more fluent than if I am speaking off the top of my head, so if he wrote his speech down on something he will be more fluent.