At about 15kg, my granddaughter has a huge reserve of stability. A child that small has a very low centre of gravity compared with an adult. Watching her in that kayak, I believe that she experiences a similar level of stability that an adult would have in a mid range adult sea kayak.

The 10 year old, who is at about the top of the weight range I designed it for, immediately felt very confident in the kayak in spite of have only a little previous experience in a TK2 with one of her parents.

If you wanted to explore this design a bit more, you can find the file on the Blue Heron Kayak forum.

http://www.blueheronkayaks.com/phpBB2/viewtopic.php?f=2&t=2299#p24004

There are a few more photos there too.

http://www.blueheronkayaks.com/phpBB2/viewtopic.php?f=6&t=2300#p24292

If you download the software and the file for this design, you could shorten it and widen it a bit. You would need a bit more volume as your son is heavier than my granddaughter. Then if you posted your version on the Kayak Foundry forum, you could get some advice about whether you have the design right.