I haven't built the Parr, but I designed and built one that is a similar shape. It is 12' long and was specifically designed for my grandaughter.

I have attached a photo of my 4 year old granddaughter and a tall 10 year old girl in the kayak. I deliberately made the cockpit small to keep oversized children from getting in the kayak. The 10 year old girl is close to the maximum size that I designed it to suit.

By the time you get yours built, your son will be slightly heavier and slightly taller, so I suspect that the 14' version might be a better choice