I can't speak to why he might not be comfortable with it, but for me it was sort of a mix of multiple different reasons. The main one is that I viewed having my sentences finished for me as basically robbing me of an opportunity to overcome the stutter on my own. I was trying many strategies and ideas to talk more fluently, so a moment where I was having trouble pronouncing a word was a perfect "practice" for better fluency. Some smaller reasons were because the corrections often ended up being not what I wanted to say, and also not wanting to be pitied.