Sorry but this seat is not safe for a 21 month old.

First of all, he should be facing backwards. His head and neck are not developed enough to withstand the impact if you're in a car accident.

Also, harnesses are usually not advised until the child is at least 4 years old. Until then, they should be a 5 point harness.

I'm not aware that there are any car seats like this in N.America. They're simply not safe!

It's useless to use a booster in a airline seat because the whole purpose is to guide the shoulder belt. Airplane seat belts don't have shoulder belts! Regardless of the "experience" of the Flight Attendant!!!