In my opinion a 154 is too big... However, a 150, although it will be a little big for him at first, will give some room to grow into it.

A 146ish is ideal right now, but you have to factor growth in if your son plans on riding it for a few seasons.

How quickly is he growing? What size foot does he have? The joystick is built as a mid-wide waist width meaning it can accommodate a large foot size (up to an 11 or 12) ... if his foot is smaller than this then it will just be a little slower edge to edge while turning but provide a great surface area (stability) for him.

You may also want to check out Burton's Hero board. Similar shape as the joystick with a less deep sidecut and a softer flex. A Hero 149 would be a perfect board for him. Even though its a little big to start, the softer flex (compared to a joystick) will make it more "fun" for him even though the length is longer. As he grows, the length will still be there!

Check it here:

http://www.dogfunk.com/snowboard/Burton-Hero-Snowboard/BUR2741M.html

Or the same board with a limited graphic choice is here:

http://www.dogfunk.com/snowboard/Burton-Coalition-The-Hero-Ltd-Snowboard/BUR3221M.html