I started our son Rob skiing when he was 2 and skateboarding when he was around 4, by the time he was 7 and I was snowboarding, he wanted to switch...the next year when he was 8, I bought a 145 burton something that had no edges for him and off we went...this is a pic I took that first time :)
With your sons Skating, Surfing and Skiing experiences to draw from, you should feel comfortable about his desire to want to get on a board and his ability to draw some fine lines
Have Fun...