In Youtube there are many videos to learn to speak in public. On the other hand, self-confidence is key, and knowledge of the subject also. When you know the subject a lot, it is very easy to talk about it.
To improve, it is good for your child to stand in front of the mirror and start talking to himself, so he can see what it looks like. He can see that behaviors are not very pleasant, or not very flashy, and so he can improve the way he communicates