English grammar is funny. Even the most learned will get confused sometimes. But as your kid is at a learning stage, he will be more than excited to enjoy using conjunctions while making sentences and will show you that he is improving in his writing skills.
What you need to know:
He might have started using easy conjunctions for now, but it is not necessary that he is aware of their meaning and why they are being used in a particular sentence. Clarify these small things to him as what you teach him now will stay with him for the rest of his life.