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What sort of kayak do YOU paddle (tour sit inside, SOT, etc.)? Does your son want to "get into" kayaking meaning is he excited, willing to get wet and have fun learning or just go out now and then?
A SOT is definitely easier if he just gets out now and then but there are some nicely scaled down models (not just shorter) that give kids a "real" kayak experience.
I've heard folks speak well of the Wilderness Systems Piccolo which seems to be no longer made but may be found used. Currently they offer the Tsunami SP which may be a good fit.
I think ideally for any boat you want to avoid one that is too wide making it hard for short arms to reach far enough for good strokes. 22" or narrower isn't a bad choice for a kid -- would be like 26" or more for adult. Narrow also makes it easier for them to play around with edging.
The most common problem I see is kids in kayaks that are much too big for them.
Most kids are quick learners, have good reflexes and balance, and are not afraid of getting wet. They want to go fast. Parents are often far more worried about stability than the kids are. A boat that's too wide and/or too deep for a small paddler will be uncomfortable and slow.
SOTs have the advantage of not having to be emptied, but they're usually slower than sit-insides. if you want to paddle with your child that may be a consideration.
Possibilities might include:
WS Tsunami SP
Perception Prodigy XS
Perception Carolina 12 XS/Umiak (discontinued)
Perception Acadia Scout(discontinued)
WS Piccolo (discontinued)
Jackson Mini Tripper
There are also build-your-own options, such as...
A skin-on-frame: http://www.yostwerks.com/SeaFlea1.html
Stitch & glue: http://www.pygmyboats.com/boats/osprey-13-kayak-kit.html