Stutter here since 2nd grade. Went to therapy through my junior year of high school, which gave me lots of fundamental tools to help me though blocks, but the best tool I've ever come across to actually work on stuttering less and processing speech slower internally had been reading children's books to my girls. I'm forced to read fairly slow to ensure they understand the book, but it also gives me a chance to slow down the speed at which im.processing the words in my mind. If he has any young nieces/nephews or if you have any family friends with young children I encourage him to give it a shot.