Remember the simple tips
Give him time to finish and do not interrupt or finish off words.
Do not comment on his speech unless you notice that he is struggling to speak, or reacting to his stammering by making a comment, or a gesture: 'Do not ignore his distress.' Give him support, as you would for any ordinary difficulty like a fall, and comment gently, 'Well done, that was a hard word for you.' A hug might also be a good idea.
Listen attentively and repeat back some part of what he said so that the child feels that what he said is more important than how he said it.
Maintain normal eye contact and do not show any impatience. For example, avoid frequently nodding; looking at a watch or surreptitiously getting on with another task while the child is speaking.
Slow your own speech with natural pauses, demonstrating that there is no need to rush.
Talk and play regularly with your child in a relaxed environment where you follow his lead as to what he wants to do.
Aim to build his self-esteem by emphasising what he does well and using his name or family nickname regularly when you talk with him so he knows that he is unique and special to you. He is more likely then to develop the confidence to manage his speaking even when stammering severely.
Try to make sure that the other children and adults he sees regularly also follow this simple advice.