First, correcting him will make it worse, he doesn't choose to stutter, it's very uncomfortable for him. Our son started stuttering a while back, and we found this site to be a good source of information (there's a lot of bad information on the web) : https://stutteringtherapyresources.com/blogs/blog/what-should-we-say-to-the-parent-of-a-young-child-who-has-recently-started-to-stutter