Here is a 10 second clip of my four yr old riding cable. He rides boat a good bit as well. He can hit the small beginner features at the park. He's doing some one wake jumps and riding switch, and has a pretty good ollie as you can see in the last second of the video. We started at 3 on the Ronix vision and it has been a great board for him. I don't regret buying it at all and it looks like it will last him for a few more seasons as he grows and progresses. Don't limit him, it's amazing how they can progress with enough water time. On the flip side, don't push him either.
Make sure to get him a helmet. We use shredready. He is small and the board is big. Your son is also very flexible at that age. If he falls on his stomach it is very easy for the board to come up from behind and hit him in the head. Get him the best protective gear you can.
There is nothing more fun to me than a day on the water with my little buddy. Riding doubles with him keeps a smile on my face all day. Have fun!