There may be many causative factors regarding stammering but person usually continues to stammer because of psychological reasons like anxiety, fear, lack of confidence etc. In fact it is a vicious cycle; person continues to stammer because these factors and these factors worsen because of stammering. Apart from Pranayama, you have to do other deep breathing exercises, Relaxation exercises, Speech exercises etc. Try to visualise yourself talking fluently and flawlessly for 15 minutes, every day while sitting in a meditation. Try not to feel ashamed about it and you will start gaining confidence faster.