The stadium seat works well but if you want him to have any kind of comfort you need to mount it on something. Find a small cooler that you can place in the rear well and strap it down. Then mount your stadium seat to the top of that. That way his legs will have some bend to them and he'll stay comfortable longer.
I have the PA 14 and take my 92lb ten year old with me all the time. He sets atop a 60qt cooler on a stadium seat. It's a little high up but it works better for our sight fishing the redfish here in Florida. We only do this in protected waters. When fishing in areas with boat traffic I use a much smaller cooler back there with the stadium seat on top.