5 yo son developed stutter since starting school

5 yo son developed stutter since starting school My son is a brother of 3 girls (quadruplet) Few months back he developed very slight stutter. We didnt pay much attention to it. Recently like a week ago. It became so persistent he barely now say a sentence without stuttering with it. Listening to him stutter really breakes my heart. We are listening to him without interrupting him and encouraging him to talk because sometimes he doesn’t talk because he is too shy to stutter. He is so smart. And his school teachers love him so much. Please give me some advice.