i absolutely despise when this or something similar happens to me but you wont see me outwardly express for a few reasons : 1. to me it signals that you view my speech as something you don't want to deal with which means you might not care or appreciate me furthering a conversation 2. the stutter isnt cause by anything in particular but it can be amplified or boosted by things like being upset 3. i understand you're not necessarily motived by malicious intent or to desire to offend me and that you are either uncomfortable, think you are doing me a kindness or are just oblivious to why this would offend me i am by no means a complete representation of everyone with a stutter we are not all of the same opinion that said a variation of my opinion is the one most commonly expressed when this question is posed as always i suggest you just bluntly ask him his opinion i promise you theres very little chance he will be offended by it in fact quite the opposite ive been asked a few times and im delighted because it shows that you care