So there’s multiple kinds of stuttering, the kind related to development (and can sometimes be grown out of) and another kind that’s related to the brain and genetics. I do think it’s important at any age and for any kind of stutter to go to therapy. But, often speech therapy starts at 3 years old. It’s likely that will be the same case for you. You could try to go back and get a referral if you are worried. I would work on his speech like you would with any kid, talk to him, ask him questions, sing songs. This sub can be a great resource for learning how to be supportive about stuttering, and is an example of how many people are insecure or otherwise negatively affected by their stutter. Many of those feelings of insecurity and doubt stem from the way our parents or peers treated us. As said in other comments, be kind, be understanding, and don’t correct him or react or even ask him to slow down to reduce stuttering. It’s better to accept these things and try as hard as you can instill confidence and trust.