It might sound like a crazy idea, but have you considered traveling with your Frontier?

We had the Frontier Clicktight for our daughter which she used (harnessed) in the car and on flights until 13 years old (no special needs, she's just very small for her age). We actually chose it in part because it could be used on aircraft.

While it’s of course a big and fairly heavy seat, it does have some advantages worth considering. First, you know he’s within the weight and height limits and will be safe; second, he’s familiar with it and won’t therefore be uncomfortable; third, it’s extremely unlikely you’ll have any issues installing it in the vehicle at your destination (which is always welcome after a flight!); fourth, you won’t have to worry about the time and money of buying a new seat; and lastly, if need be, he could sit in it at the airport when you’re waiting which might be more comfortable (and probably cleaner!) than a departure lounge/restaurant seat.

Obviously it’s size and weight aren’t ideally suited to traveling, but there are compact and collapsible wheeled carts you can use (for example or ) which actually make it fairly agreeable to move around.

If taking the Frontier isn’t an option and he weighs 40lbs or more, then the Mifold and BubbleBum could be used as they are both fully approved for use in the U.S. from the age of 4 years and 40lbs (and there are no other specific regulations according to California's car seat laws). It’s worth checking to see if he passes the five-step test, as children who are used to only traveling in a 5-point harness tend to need time to adjust to riding in a booster seat (even our daughter needed reminding a few times when she started at 13!).

Hopefully this is helpful!
(should you have any questions about traveling with the Frontier don’t hesitate to ask.)