A child of such age often might go through a phase called Developmental Stuttering, that is common at this age. It occurs when a child have so much to say but finds it difficult to speak out all. This might come and go or last for few months.
Next Steps
Since you are worried please consult a Speech Language Pathologist
Health Tips
Some tips:
Focus on what your child has to speak rather than his stammering.
Be supportive and wait for your child to express.
If you are a fast speaker slow down your speech while talking to your child.