Speaking in front of the public is one of my greatest fears; hence, I want to ask you, does your child really want to engage in that kind of activity? Because if he doesn't have his heart in it, then you might just be putting him into so much pressure. I do not want to discourage you, though. I just want you to listen to what your child really wants to pursue.
But keeping that aside, acquiring good public-speaking skill is quite tough, as this requires huge amount of focus, motivation, and of course, confidence to stand in front of the public. If he's into it, then show your full support by being there to listen and to act as "the public" when needed be. Typically, parents do not want to hurt their child's feelings; hence, they often overlook their mistakes and eventually fail to recognize that there are areas for improvement. I suggest you avoid doing this when watching him speak because the public will not look at your child in the same way you look at him. Teach him how he should present himself, where and how he should look at people, and how should his movements be when delivering his message. There really isn't a straight path to being a good public speaker. It all comes with practice, and not giving up during times of failure.