When kids start to walk, they often tremble and fall down. That is normal. They take time to learn to walk confidently. We only worry if they keep on trembling after some time. Same applies to speech. As a parents, when he stutter, you shouldn't let him know that he is stammering because that makes him more conscious about his speech. And you should monitor the stammer and get help if it is getting worse, ideally before he is 5.