"What car is he in?" is just my example sentence. I always hear people say you can't use a preposition at the end of a sentence, but an English Composition teacher in college taught me that if a preposition (actually word that is usually a preposition) is used like that, it takes on the form of an adverb. People good with English, is this true? I'm just curious because I still hear so many people correcting it.
I don't remember what our book said if anything.

It is accepted to use that phrase colloquially. If you were to use formal English, you would say, "In which car is he?"

"In which car is he?" would be a better way to put it.

I'd say it was an adverb, because where is he? He's in the car. So in would be an adverb of place.

It's a preposition. 'Correct' English would have the sentence as "In which car is he?".