You should consult with nearest speech therapist. They will do speech assessment. They will tell you what to do. It may be normal non fluency. There is no role of psychologist in this case.

Consult SLP or child psycologist..

I suggest better take an appointment with Speech Language Pathologist (Speech Therapy). Discuss your child problem clearly. Then you take your son for consultation.

There is very thin boundary between normal non fluency and stammering. Most of the time stammering start from the age 2-5 years. If history of stammering is more than 6 months than only we can say confidently it is stammering only. Talk to the child slowly and don't give any negative comments about his speech. Consultant Speech Languages Pathologist (SLP) for detail evaluation.

I suggest you to consult child psychologist too. It must be due to some kind of anxiety or depression which requires intervention now rather than it becomes a permanent problem.