We started with the standard kids Connley doubles with the rear toe strap. This worked great for a starter.
We then looked for a 42 or 43 after seeking the same advice as yourself. At the speeds he will be skiing these sizes will work well as it will keep him on top of the water, then as he gets older and heavier the ski should start to work better. We are hoping that the ski will last for a long time.
The only issue is that they will probably want to go faster than their ability or at least that what we found.