Practice, practice, practice. I cannot tell you how many speeches, presentations and interviews I have absolutely aced solely from rehearsing in front of a mirror. Albeit nerve wracking AF during your practice, you condition yourself to say those words/sounds/syllables when the time because you wire your brain in the sense that IT KNOWS that you have said those words fluently to yourself when rehearsing. ​ It's sort of like muscle memory but with your tongue and voice. Of course this is not a fool-proof solution, but speaking out loud to yourself/a mirror/or a small audience will really make a huge difference. ​ I wish him luck and as a stutterer, appreciate you wanting to help him.