My advice would be to not worry too much. Since he is still young, it is common for children to stutter when at that age of first learning to speak. You are doing the right thing by giving him a comfortable environment for him to speak without fear in. Chances are very likely he will outgrow it. If you want to help him out for the time being, you could possibly look into finding a highly trained speech pathologist in your area. But try not to worry too much, he is still young and his brain still has a long way to go until it fully develops