Wondering if you have to have a car seat on the plane for a 2 1/2 year old? He had to have a ticket this time. Last time, he was 10 months old and a "lap child" and had no seat. Do they require one now or is the seat belt fine? I thought they didn't have to have one, but then I see all these stickers on the seats saying they're okay for air travel, so wasn't sure.