Stammering can happen at an early age. Not all children grow out of it. But as a parent you can surely give effort and help your child cope up stammering.
In an exciting fun environment get your child describe the moment he or she is enjoying.
Have your child talk about a picture in a simple easy way.
Talk over the phone with your child. Let her/him use slow speech to talk to you and describe situation.
Make finger puppet and start a conversation.
Tell your child a joke with frequent pause and then ask him to say it in similar manner. It reinforces a slow pace of speech and pause admits of giggles.
Do not interrupt criticize or disrupt your child's conversation. Children who stutter find it easier to talk when people are listening to them and interrupting less.
They speak more freely when expressing their ideas rather than answering of questions asked by adults. Rather than questioning too much you can simple comment on what they said. This way it will help him to understand that you have heard him.
Lastly convey that you accept your child as he is and will support him all the way.