Possible cause in this case may be one of d followings
1. Fear and anxiety: if a child is always shouted down and scolded when s/he speaks this may lead to d child having a poor self image, loss of confidence in self they by affecting the child's normal speech pattern. As a result a child may begin to stutter or go mute.
2. Imitation: is there an adult or child they like so much who stutters? Parent should try and observe d child in order to address the reason for the stuttering.

What to do
1. Find out and address the cause of stuttering
2. Build on the child's self perception and confidence. Let him know they are cool the way they are without stuttering
3. Notice times d child didn't stutter and reinforce the child 4 d clear sentence or statement
4. When stuttering, touch ur child on his shoulder calmly n slightly and ask the child to speak slowly and calmly that you are listening.
5. Encourage ur location child speak slowly rather than rush through words

If stuttering persists after doing all you can pls see a speech pathologist/therapist