It's usually best to start out on your belly when you're a beginner. Just pull him up slowly and keep the speed under 20mph. He should be able to get up on his knees at around 15mph. Now what your son needs to do is to lay on the board with his forearms on the front of the board. When the boat pull comes, his nose should be right above the tip of the kneeboard. He'll bounce if he's too far back. When he's ready to get on his knees, have him grab the tip of the board with both hands, while still holding onto the handle. Then he just needs to pop up on his knees.
For more instruction and video, check out: http://www.kneeboardplanet.com