I teach kids every weekend for a local surf shop. Eight is a great age, don't wait any longer since he wants to do it. Take him out the first few times with little ankle slappers. A day its not too cold, etc. Bring him somewhere you can stand and give him a push. Start with a funshape sized foamy, unless he's a real big kid longboards will have too much float and no maneuverability for him. Costco sells some foamys for $99. You have time to get him ripping on a shortboard but its got to start out fun. Its always good to have a foamy in your quiver for first timers so start there...