You forgot to mention your son's height and weight..
Another option (rather than bying one board for the both of you) is to get two older boards (used or new) for the price of one 2006 board. I'd recommend checking out
http://www.wakeworld.com/BoardGuide/ and then doing a web search to find a best deal on whatever you find in the Board Guide.
And if you are gonna get just one board, consider one of those boards that have three removable fins on each end so you can mix and match various fin sizes and shapes to get the most out of that one board.