My Radian is great for two things: 1) extended rear-facing and 2) fitting three seats across a car. Although it folds and comes with backpack straps, it is HEAVY! The frame is steel, and it feels like it! It is also really expensive for use just as a travel seat.
The only way I would recommend a Radian is if you plan to rear-face your son again in it. If you plan for him to stay forward-facing, then I would opt for something lighter weight, like an Evenflo Maestro. That one in particular has the same top harness heights as your Nautilus, goes to 50lbs, and my understanding is that it installs well on airplanes, allowing the tray table to come down and be used.
You could bungee it to an umbrella stroller & push it through the airport, then gate check the stroller and use the carseat on board.
I hope that helps!