My solution to that problem was to design my own.

I wanted a kayak for my granddaughter who was about three at the time. I wanted my design to suit a child between about 15kg and 25kg.

I have photos of my granddaughter and a tall, slim ten year old girl who weighs about 25kg that suggest it works in that size range. It sounds as if your son is a bit heavier than those girls at the same age.

It is a bit longer than you are planning, but I wanted it to be efficient. Making it shorter would mean it had to be wider to get the volume and the stability needed. At 12 feet long, it looks and performs like a serious kayak.

I used Kayak Foundry from the Blue Heron Kayaks site to design it.