Are you talking about teaching him to ski on one ski? Does he know how to ski on two? If it's one ski, don't go too small, use the biggest ski he can control in the water. The more surface area you can start with, the easier it will be. I've taught dozens of kids to get up on a slalom, and I get back there with them on a pair of old trick skis, and hold their jacket to steady them while they learn what it feels like to get up. Once they are up and skiing a little bit, starts are much, much easier.