Two things to consider the first being that kids learn and progress fast so the basic boards don't really last long. The flipside is you will always need a "boat Board"
The second is that you will probably pay $300.00 for a wakeboard brand wakesurf board, the high end advanced boards from folks like Shred Stixx, Soulcraft, Brigade, Walker etc etc usually run from $6-800 and if he enjoys it you will wind up there anyway.
If you still want to go your original route, check out sites like EVO, the have a very nice Ronix Koal for a great price right now. It will serve you well.