I believe I read another post from you, please forgive me if I am wrong. You were using your infant seat that he has outgrown on the plane because of turbulence, but did not want to use it with the car because technically he has outgrown it?
You could always bring his regular, full size convertible carseat onto the plane. Most (not all) are FAA approved, so it could be installed in the plane and then in the car. I'm traveling with an 8 month old in 2 weeks, so I definitely understand the headaches of lugging a full size carseat. Back in December, I purposely bought an infant seat with a higher weight and height limit just for our September trip. He thought I was bonkers, but now I'm really glad for it! In the past, we have brought the convertible seat for our dd. We used bungee cords to strap it to a luggage cart, then I strapped her into the seat. It was a makeshift stroller! I have also heard of people strapping (or wedging) their convertible carseats into their regular strollers. It is always so difficult at the airport to juggle luggage, carryons, strollers, carseats, children, etc. I can barely keep up with my kids at the grocery store!
Good luck,
Jennifer