He can see if you are frightened or confused by this. Little children just take things in stride until others show them that there is something to fear, or worry about, or be embarrassed by. If he asks a question, answer it. Never assume that he won't understand. He'll know if you're hiding something from him.
People usually slur it when they say it.
Tell him that everything a person does is controlled by the brain. Sometimes the orders that the brain gives get confused or mixed up by the time they get to the rest of the body. So the body does things a little differently than he might expect. It's important that he knows what to tell other people if he is asked. If he can give another child a simple explanation to answer any questions that they might ask, then he will feel confident that he understands the problem himself and will not feel inadequate.