Do i need to replace the whole engine if it is knocking?

I have a 2002 Chevy Malibu. recently the knock has gotten a lot worse. when i had my oil change, they put some additive in it that took away the knock for a while(or lessened it) anyways the knock came back worse than ever. does the whole engine need to be replaced? i had a mechanic "look" at it. he sorta just listened to it. he said the engine needs to be replaced, bring it to him for 500 dollars. should i trust him or do more research?? thanks